BLACKROCK FUNDS III

It is proposed that this filing will become effective (check appropriate box)

Beginning on January 1, 2021, as permitted by regulations adopted by the Securities and Exchange Commission, paper copies of the Fund’s shareholder reports will no longer be sent by mail, unless you specifically request paper copies of the reports from BlackRock or from your financial intermediary, such as a broker-dealer or bank. Instead, the reports will be made available on a website, and you will be notified by mail each time a report is posted and provided with a website link to access the report.

You may elect to receive all future reports in paper free of charge. If you hold accounts directly with BlackRock, you can call (888) 204-3956 to inform BlackRock that you wish to continue receiving paper copies of your shareholder reports. If you hold accounts through a financial intermediary, you can follow the instructions included with this disclosure, if applicable, or contact your financial intermediary to request that you continue to receive paper copies of your shareholder reports. Please note that not all financial intermediaries may offer this service. Your election to receive reports in paper will apply to all funds advised by BlackRock Advisors, LLC, BlackRock Fund Advisors or their affiliates, or all funds held with your financial intermediary, as applicable.

If you already elected to receive shareholder reports electronically, you will not be affected by this change and you need not take any action. You may elect to receive electronic delivery of shareholder reports and other communications by contacting your financial intermediary, if you hold accounts through a financial intermediary. Please note that not all financial intermediaries may offer this service.

This Prospectus contains information you should know before investing, including information about risks. Please read it before you invest and keep it for future reference.

The Securities and Exchange Commission has not approved or disapproved these securities or passed upon the adequacy of this Prospectus. Any representation to the contrary is a criminal offense.

The investment objective for BlackRock Cash Funds: Treasury (the “Fund”), a series of BlackRock Funds III (the “Trust”), is to seek current income as is consistent with liquidity and stability of principal.

This table describes the fees and expenses that you may pay if you buy and hold Capital Shares of BlackRock Cash Funds: Treasury.

This Example is intended to help you compare the cost of investing in Capital Shares of the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in Capital Shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that Capital Shares’ operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

BlackRock Cash Funds: Treasury seeks to achieve its investment objective by investing at least 99.5% of its total assets in cash, U.S. Treasury bills, notes and other direct obligations of the U.S. Treasury, and repurchase agreements secured by such obligations or cash. The Fund invests in securities maturing in 397 days or less (with certain exceptions) and the portfolio will have a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less. In addition, the Fund may invest in variable and floating rate instruments and transact in securities on a when-issued, delayed delivery or forward commitment basis.

agreements secured by such obligations. This policy is a non-fundamental policy of the Fund and the Fund will not change the policy without providing shareholders with at least 60 days’ prior notice of any change in the policy.

U.S. Treasury obligations are backed by the full faith and credit of the U.S. Government. The principal and interest of all securities held by the Fund are payable in U.S. dollars.

The securities purchased by the Fund are subject to the quality, diversification, and other requirements of Rule 2a-7 under the Investment Company Act of 1940, as amended (the “1940 Act”), and other rules of the Securities and Exchange Commission. The Fund will only purchase securities that present minimal credit risk as determined by the Fund’s investment adviser, BFA, pursuant to guidelines approved by the Trust’s Board of Trustees.

The Fund is a “feeder” fund that invests all of its investable assets in Treasury Master Portfolio of MIP, which has the same investment objective and strategies as the Fund. All investments are made at the Treasury Master Portfolio level. This structure is sometimes called a “master/feeder” structure. The Fund’s investment results will correspond directly to the investment results of Treasury Master Portfolio. For simplicity, the prospectus uses the name of the Fund or the term “Fund” (as applicable) to include Treasury Master Portfolio.

Risk is inherent in all investing. You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time. The following is a summary description of the principal risks of investing in the Fund. The order of the below risk factors does not indicate the significance of any particular risk factor.

There are currently no Capital Shares of BlackRock Cash Funds: Treasury outstanding as of the date of this prospectus. As a result, the chart, the table and the seven-day yield shown below give you a picture of the performance for SL Agency Shares of the Fund, which are not offered in this prospectus. The performance of the Fund’s Capital Shares would be substantially similar to SL Agency Shares because Capital Shares and SL Agency Shares are invested in the same portfolio of securities and performance would only differ to the extent that Capital Shares and SL Agency Shares have different expenses. The actual returns and seven-day yields of Capital Shares would have been lower than those of the SL Agency Shares because Capital Shares have higher expenses than the SL Agency Shares.

The bar chart and table in this section provide some indication of the risks of investing in BlackRock Cash Funds: Treasury by showing the changes in the Fund’s performance from year to year. The bar chart shows the returns of SL Agency Shares of the Fund for each complete calendar year since the inception of the Fund’s SL Agency Shares. The average annual total return table compares the average annual total return of SL Agency Shares of the Fund to that of the Money Fund Report (“MFR”) Treasury and Repo Institutional Average, a service of iMoneyNet. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. How the Fund performed in the past is not necessarily an indication of how it will perform in the future. If BFA and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. The Fund is a money market fund managed pursuant to the requirements of Rule 2a-7 under the 1940 Act. Effective May 28, 2010, Rule 2a-7 was amended to impose new liquidity, credit quality and maturity requirements on all money market funds. Fund performance shown prior to May 28, 2010 is based on 1940 Act rules then in effect and is not an indication of future returns. Updated information on the Fund’s performance can be obtained by visiting http://www.blackrock.com/cash or can be obtained by phone at 1-888-204-3956.

During the ten-year period shown in the bar chart, the highest return for a quarter was 0.59% (quarter ended June 30, 2019) and the lowest return for a quarter was 0.00% (quarter ended June 30, 2015).

Current Yield: You may obtain the Fund’s current 7-day yield by calling 1-888-204-3956 (toll-free) or by visiting the Fund’s website at www.blackrock.com/cash.

Treasury Master Portfolio’s investment adviser is BlackRock Fund Advisors (previously defined as “BFA”).

The minimum initial investment for Capital Shares of BlackRock Cash Funds: Treasury is $25 million, although the Fund may reduce or waive the minimum in some cases. You may generally purchase or redeem shares of the Fund each day the bond markets and the Fedwire Funds Service are open. To purchase or sell shares you should contact your financial professional or selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, Inc. and its affiliates) (each, a “Financial Intermediary”), or, if you hold your shares through the Fund, you should contact the Fund by phone at 1-888-204-3956, or by mail to State Street Bank and Trust Company, P.O. Box 5493, Boston, Massachusetts 02206.

You may purchase or sell shares without paying a sales charge. Your order to purchase or sell shares must be received in proper form, as determined by State Street Bank and Trust Company, the Fund’s transfer agent, or a Financial Intermediary pursuant to an appropriate agreement, by 5:00 p.m. Eastern time on any day the Fund is open (a “Business Day”) (or, if the Fund closes early, by such closing time) to purchase or sell shares at that day’s net asset value. Orders received after 5:00 p.m. Eastern time on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

The Fund’s dividends and distributions may be subject to U.S. federal income taxes and may be taxed as ordinary income or capital gains, unless you are a tax-exempt investor or are investing through a qualified tax-exempt plan described in Section 401(a) of the Internal Revenue Code of 1986, as amended, in which case you may be subject to U.S. federal income tax upon withdrawal from such tax deferred arrangements.

If you purchase shares of BlackRock Cash Funds: Treasury through a Financial Intermediary, the Fund and BlackRock Investments, LLC, the Fund’s distributor, or its affiliates may pay the Financial Intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the Financial Intermediary and your individual financial professional to recommend the Fund over another investment.

Ask your individual financial professional or visit your Financial Intermediary’s website for more information.

The Fund is a “feeder” fund that invests all of its assets in Treasury Money Market Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio (“MIP”), a mutual fund that has an investment objective and strategies substantially identical to those of the Fund. All discussion of the investment objective, strategies and risks of the Fund refers also to the investment objective, strategies and risks of the Master Portfolio, unless otherwise indicated. A description of the relationship of the Fund to the Master Portfolio appears below under the heading “Account Information — Master/Feeder Mutual Fund Structure.”

The Fund is a government money market fund managed pursuant to Rule 2a-7 under the 1940 Act.

The Trust’s Board of Trustees (the “Board”) has chosen not to subject the Fund to liquidity fees or redemption gates due to declines in the Fund’s weekly liquid assets.

The investment objective for the Fund is to seek current income as is consistent with liquidity and stability of principal.

The Fund seeks to achieve its investment objective by investing at least 99.5% of its total assets in cash, U.S. Treasury bills, notes and other direct obligations of the U.S. Treasury, and repurchase agreements secured by such obligations or cash.

The Fund will invest, under normal circumstances, at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in U.S. Treasury bills, notes and other obligations of the U.S. Treasury, and repurchase agreements secured by such obligations. This policy is a non-fundamental policy of the Fund and the Fund will not change the policy without providing shareholders with at least 60 days’ prior notice of any change in the policy.

The Fund invests in securities maturing in 397 days or less (with certain exceptions) and the portfolio will have a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less. In addition, the Fund may invest in variable and floating rate instruments and transact in instruments on a when-issued, delayed delivery or forward commitment basis. The principal and interest of all securities held by the Fund are payable in U.S. dollars. The Master Portfolio may transfer uninvested cash balances into a single joint account at the Master Portfolio’s custodian bank, the daily aggregate balance of which will be invested in one or more repurchase agreements.

The following is a description of certain risks of investing in the Fund. The order of the below risk factors does not indicate the significance of any particular risk factor.

Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.

Income Risk — The Fund’s yield will vary as the short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates.

Interest Rate Risk — Interest rate risk is the risk that the value of a debt security may fall when interest rates rise. In general, the market price of debt securities with longer maturities will go up or down more in response to changes in interest rates than the market price of shorter-term securities. Due to fluctuations in interest rates, the market value of such securities may vary during the period shareholders own shares of the Fund.

Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. The value of a security or other asset may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security or other asset, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issue, recessions, or other events could have a significant impact on the Fund and its investments. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

Repurchase Agreements Risk — If the other party to a repurchase agreement defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security and the market value of the security declines, the Fund may lose money.

Stable Net Asset Value Risk — The Fund may not be able to maintain a stable NAV of $1.00 per share at all times. If the Fund fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Fund, along with other money market funds, could be subject to increased redemption activity.

Treasury Obligations Risk — Direct obligations of the U.S. Treasury have historically involved little risk of loss of principal if held to maturity. However, due to fluctuations in interest rates, the market value of such securities may vary during the period shareholders own shares of the Fund.

U.S. Government Obligations Risk — Not all U.S. Government securities are backed by the full faith and credit of the United States. Obligations of certain agencies, authorities, instrumentalities and sponsored enterprises of the U.S. Government are backed by the full faith and credit of the United States (e.g., the Government National Mortgage Association); other obligations are backed by the right of the issuer to borrow from the U.S. Treasury (e.g., the Federal Home Loan Banks) and others are supported by the discretionary authority of the U.S. Government to purchase an agency’s obligations. Still others are backed only by the credit of the agency, authority, instrumentality or sponsored enterprise issuing the obligation. No assurance can be given that the U.S. Government would provide financial support to any of these entities if it is not obligated to do so by law.

Variable and Floating Rate Instrument Risk — Variable and floating rate securities provide for periodic adjustment in the interest rate paid on the securities. These securities may be subject to greater illiquidity risk than other fixed income securities, meaning the absence of an active market for these securities could make it difficult for the Fund to dispose of them at any given time.

When-Issued and Delayed Delivery Securities and Forward Commitments Risk — When-issued and delayed delivery securities and forward commitments involve the risk that the security the Fund buys will lose value prior to its delivery. There also is the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Fund may lose both the investment opportunity for the assets it set aside to pay for the security and any gain in the security’s price.

The Fund may also be subject to certain other non-principal risks associated with its investments and investment strategies, including:

Cyber Security Risk — Failures or breaches of the electronic systems of the Fund, the Fund’s adviser, distributor, and other service providers, or the issuers of securities in which the Fund invests have the ability to cause disruptions and negatively impact the Fund’s business operations, potentially resulting in financial losses to the Fund and its shareholders. While the Fund has established business continuity plans and risk management systems seeking to address system breaches or failures, there are inherent limitations in such plans and systems. Furthermore, the Fund cannot control the cyber security plans and systems of the Fund’s service providers or issuers of securities in which the Fund invests.

Expense Risk — Fund expenses are subject to a variety of factors, including fluctuations in the Fund’s net assets. Accordingly, actual expenses may be greater or less than those indicated. For example, to the extent that the Fund’s net assets decrease due to market declines or redemptions, the Fund’s expenses will increase as a percentage of Fund net assets. During periods of high market volatility, these increases in the Fund’s expense ratio could be significant.

Illiquid Investments Risk — The Fund’s illiquid investments may reduce the returns of the Fund because it may be difficult to sell the illiquid investments at an advantageous time or price. The Fund may be unable to pay redemption proceeds within the time period stated in this prospectus because of unusual market conditions, an unusually high volume of redemption requests, or other reasons.

Investment in Other Investment Companies Risk — As with other investments, investments in other investment companies, including exchange-traded funds, are subject to market and selection risk. In addition, if the Fund acquires shares of investment companies, including ones affiliated with the Fund, shareholders bear both their proportionate share of expenses in the Fund (including management and advisory fees) and, indirectly, the expenses of the investment companies (to the extent not offset by BFA through waivers). To the extent the Fund is held by an affiliated fund, the ability of the Fund itself to hold other investment companies may be limited.

For a description of the Fund’s policies and procedures with respect to disclosure of the Master Portfolio’s portfolio holdings, and a discussion of the Fund’s investments and risks, please refer to the Fund’s combined Statement of Additional Information (“SAI”).

The minimum initial investment amount for Capital Shares of the Fund is $25 million; however, in certain situations, this minimum initial investment amount may be reduced or waived. Please contact your shareholder servicing agent (“Shareholder Servicing Agent”) or State Street Bank and Trust Company, the Fund’s transfer agent (the “Transfer Agent”), for more information.

The Fund offers additional share classes with different expenses and expected returns, including share classes you may be eligible to purchase. Call 1-888-204-3956 (toll-free) for additional information.

In order to invest, a completed account application form must be submitted to, and processed by, your Shareholder Servicing Agent or the Transfer Agent and an account number assigned. You may be asked to provide information to verify your identity when opening an account.

Your Shareholder Servicing Agent may charge you a fee and may offer additional account services. Additionally, your Shareholder Servicing Agent may have procedures for placing orders for Capital Shares that differ from those of the Fund, such as different investment minimums or earlier trading deadlines. Please contact your Shareholder Servicing Agent directly for more information and details.

Shares of the Fund are only registered for sale in the United States and certain of its territories. Consequently, the Fund generally does not accept investments from non-U.S. residents.

The Fund is generally open Monday through Friday and is closed on weekends and generally closed on all other days that the primary markets for the Master Portfolio’s portfolio securities (i.e., the bond markets) are closed or the Fedwire Funds Service is closed. The holidays on which both the Fedwire and the bond markets are closed currently are: New Year’s Day, Martin Luther King, Jr. Day, Presidents’ Day, Memorial Day, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving Day and Christmas Day. The Fund is also closed on Good Friday.

To purchase shares of the Fund, you should contact your financial professional or selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, Inc. (“BlackRock”) and its affiliates) (each, a “Financial Intermediary”), or, if you hold your shares through the Fund, you should contact the Fund by phone at 1-888-204-3956, or by mail to State Street Bank and Trust Company, P.O. Box 5493, Boston, Massachusetts 02206. Orders must be received prior to the next NAV cut-off time to receive that NAV.

Your purchase order must be received in proper form by the Fund or State Street Bank and Trust Company prior to the deadlines noted below on any day the Fund is open (a “Business Day”) (or, if the Fund closes early, at such closing time) to receive that NAV. However, the Fund may also honor a purchase order if the Fund can verify that the purchase order was submitted to a Financial Intermediary that is an authorized agent of the Fund before the applicable deadline.

The Fund will open for business and begin accepting purchase orders at 8:30 a.m. (Eastern time) on any Business Day. Purchase orders for the Fund’s Capital Shares must be received by 5:00 p.m. (Eastern time) on any Business Day.

Notwithstanding the foregoing, on any day that the principal bond markets close early (as recommended by The Securities Industry Financial Markets Association (“SIFMA”)) or the Federal Reserve Bank of Philadelphia or the New York Stock Exchange (the “NYSE”) closes early¹, the Fund may advance the time on that day by which a purchase order must be placed so that it will be effected and begin to earn dividends that day.

Orders received after the applicable deadline for the Fund on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

Payment for Capital Shares of the Fund may be made only in federal funds or other immediately available funds. You may be charged for any costs incurred, including any costs incurred to recompute the Fund’s NAV, in connection with a purchase order that has been placed but for which the Fund has not received full payment by the close of the federal funds wire (normally 6:00 p.m. Eastern time) on the day the purchase order was placed. This payment deadline may be extended by one Business Day where a purchase order is processed through certain electronic platforms where same-day cash settlement is impracticable. The Fund will notify a shareholder or Financial Intermediary if its purchase order or payment was not received by an applicable deadline.

The Fund reserves the right to suspend or discontinue the offer and sale of its shares and reject or cancel any purchase order for any reason.

To redeem shares of the Fund, you should contact your Financial Intermediary or financial professional, or, if you hold your shares through the Fund, you should contact the Fund by phone at 1-888-204-3956, or by mail to State Street Bank and Trust Company, P.O. Box 5493, Boston, Massachusetts 02206. Redemption orders may be placed either in number of shares or in dollars.

The Fund will open for business and begin accepting redemption orders at 8:30 a.m. (Eastern time) on any Business Day. The Fund will accept redemption orders until 5:00 p.m. (Eastern time) on any Business Day.

Your redemption order must be received in proper form by State Street Bank and Trust Company or your Shareholder Servicing Agent pursuant to an appropriate agreement by the applicable deadline (or, if the Fund closes early, by such closing time) to sell shares at that NAV. Orders received after the applicable deadline for the Fund on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

If redemption orders are received by State Street Bank and Trust Company on a Business Day by the established deadlines, payment for redeemed Fund shares will typically be wired in federal funds on that same day. The Fund can delay payment for one Business Day.

If you purchased shares through a Financial Intermediary that entity may have its own earlier deadlines for the receipt of the redemption order.

Where a redemption order is processed through certain electronic platforms where same-day cash settlement is impracticable, payment for redeemed shares will generally be delayed by one Business Day. If the Federal Reserve Bank of Philadelphia is closed on the day the redemption proceeds would otherwise be wired, wiring of the redemption proceeds may be delayed by one additional Business Day.

Notwithstanding the foregoing, on any day that the principal bond markets close early (as recommended by SIFMA) or the Federal Reserve Bank of Philadelphia or the NYSE closes early, the Fund may advance the time on that day by which a redemption order must be placed so that it will be effected that day.

The Fund reserves the right to redeem your shares automatically and close your account for any reason, subject to applicable law, and send you the proceeds. For example, the Fund may redeem your shares automatically to reimburse the Fund for any losses sustained by reason of your failure to make full payment for shares purchased or to collect any charge relating to a transaction effected for your benefit that is applicable to the Fund’s shares, as provided from time to time in this prospectus. In addition, the Fund reserves the right to send your redemption proceeds in the form of securities from the Master Portfolio. Any such redemption shall be effected at the NAV next determined after the redemption order is entered.

Under normal and stressed market conditions, the Fund typically expects to meet redemption requests by using cash or cash equivalents in its portfolio or by selling portfolio assets to generate additional cash.

A direct buyer who has established an account with the Fund can add to or redeem from that account by phone or through the mail.

Upon receipt of a proper redemption request submitted in a timely manner and otherwise in accordance with the redemption procedures set forth in this prospectus, the Fund will redeem the requested shares and make a payment to you in satisfaction thereof no later than the Business Day following the redemption request.

The Fund may postpone and/or suspend redemption and payment beyond one Business Day only as follows:

If the Board, including a majority of the non-interested Trustees, determines either that (1) the Fund has invested, at the end of a business day, less than 10% of its total assets in weekly liquid assets, or (2) the Fund’s calculated NAV per share has deviated from $1.00 or such deviation is likely to occur; then the Board, subject to certain conditions, may in the case that the Board has determined to liquidate the Fund irrevocably, suspend redemptions and payment of redemption proceeds in order to facilitate the permanent liquidation of the Fund in an orderly manner. If this were to occur, it would likely result in a delay in your receipt of your redemption proceeds.

The Fund reserves the right to advance the time for accepting purchase or redemption orders on any day when the NYSE, bond markets (as recommended by SIFMA) or the Federal Reserve Bank of Philadelphia closes early, trading on the NYSE is restricted, an emergency arises or as otherwise permitted by the SEC. See “How to Buy Shares” and “How to Sell Shares” for further information. In addition, the Board may, for any Business Day, decide to change the time as of which the Fund’s NAV is calculated in response to new developments such as altered trading hours, or as otherwise permitted by the SEC.

In the event the bond markets do not open for business because of an emergency or other unanticipated event, the Fund may, but is not required to, open for purchase or redemption transactions if the Fedwire Funds Service is open. To learn whether the Fund is open for business during an emergency or an unanticipated bond market closing, please call (800) 441-7450.

Under certain circumstances, if no activity occurs in an account within a time period specified by state law, a shareholder’s shares in the Fund may be transferred to that state.

The Fund has adopted a shareholder servicing plan for certain share classes of the Fund (the “Plan”) that allows the Fund to pay shareholder servicing fees for certain services provided to its shareholders for such share classes.

The shareholder servicing fees payable pursuant to the Plan are paid to compensate Financial Intermediaries for the administration and servicing of shareholder accounts and are not costs which are primarily intended to result in the sale of the Fund’s shares. Because the fees paid by the Fund under the Plan are paid out of the Fund’s assets on an ongoing basis, over time these fees will increase the cost of your investment in the shares of the Fund and may cost you more than paying other types of sales charges.

In addition to shareholder servicing fees that the Fund may pay to a Financial Intermediary pursuant to the Plan and fees the Fund pays to its transfer agent, BFA, on behalf of the Fund, may enter into non-Plan agreements with affiliated and unaffiliated Financial Intermediaries pursuant to which the Fund will pay a Financial Intermediary for administrative, networking, recordkeeping, sub-transfer agency, sub-accounting and/or shareholder services. These non-Plan payments are generally based on either (1) a percentage of the average daily net assets of Fund shareholders serviced by a Financial Intermediary or (2) a fixed dollar amount for each account serviced by a Financial Intermediary. The aggregate amount of these payments may be substantial.

From time to time, BFA, the Fund’s distributor or their affiliates also may pay a portion of the fees for administrative, networking, recordkeeping, sub-transfer agency, sub-accounting and shareholder services described above at its or their own expense and out of its or their profits. BFA, the Fund’s distributor and their affiliates may also compensate affiliated and unaffiliated Financial Intermediaries for the sale and distribution of shares of the Fund. These payments would be in addition to the Fund payments described in this prospectus and may be a fixed dollar amount, may be based on the number of customer accounts maintained by the Financial Intermediary, may be based on a percentage of the value of shares sold to, or held by, customers of the Financial Intermediary or may be calculated on another basis. The aggregate amount of these payments by BFA, the Fund’s distributor and their affiliates may be substantial and, in some circumstances, may create an incentive for a Financial Intermediary, its employees or associated persons to recommend or sell shares of the Fund to you.

Please contact your Financial Intermediary for details about payments it may receive from the Fund or from BFA, the Fund’s distributor or their affiliates. For more information, see the SAI.

The price you pay when you purchase or redeem the Fund’s shares is the NAV next determined after confirmation of your order.

The share price of Capital Shares of the Fund is calculated by dividing the value of the net assets of the Fund attributable to Capital Shares (i.e., the value of its total assets less total liabilities) by the total number of outstanding shares of such share class. The NAV per share is generally rounded to the nearest cent for the Fund.

The Fund’s NAV per share is calculated on each Business Day based on the NAV of the Master Portfolio. Generally, trading in U.S. Government securities, short-term debt securities, and money market instruments is substantially completed each day at various times prior to the close of business on the NYSE. The values of such securities used in computing the NAV of the Master Portfolio’s shares are determined as of such times.

In computing NAV, the Master Portfolio uses the amortized cost method of valuation, meaning that the calculation is based on a valuation of the assets held by the Master Portfolio at cost, with an adjustment for any discount or premium on a security at the time of purchase. The Fund’s SAI includes more information about the methods for valuing the Master Portfolio’s investments.

The NAV of the Fund is determined on each Business Day as of 5:00 p.m. Eastern time (or, if the Fund closes early, at such closing time).

The Fund declares distributions of its net investment income daily and distributes them monthly to shareholders. The Fund distributes its net realized capital gains, if any, to shareholders at least annually. Distributions payable to you by the Fund will be automatically reinvested in additional shares of the Fund unless you have elected to receive distribution payments in cash.

You begin earning distributions on your shares the day your purchase order takes effect. You continue earning daily distributions on your shares up to but not including the date you sell them.

The Fund credits distributions earned on weekends and holidays to the preceding Business Day. If you sell shares before the monthly distribution payment date, the Fund remits any distributions declared but not yet paid on the next distribution payment date. If you sell all shares before the monthly distribution payment date, the Fund remits all distributions accrued with the sale proceeds.

Market timing is an investment technique involving frequent short-term trading of mutual fund shares designed to exploit market movements or inefficiencies in the way a mutual fund prices its shares. The Boards of Trustees of the Trust and MIP have evaluated the risks of market timing activities by the Fund’s shareholders and have determined that due to (i) the nature of the Fund’s portfolio holdings; (ii) the nature of the Fund’s shareholders; and (iii) the Fund’s policy of seeking to maintain the Fund’s NAV per share at $1.00 each day, it is unlikely that (a) market timing would be attempted by the Fund’s shareholders or (b) any attempts to market time the Fund by shareholders would result in a negative impact to the Fund or its shareholders. As a result, the Boards of Trustees of the Trust and MIP have not adopted policies and procedures to deter short-term trading in the Fund. There can be no assurances, however, that the Fund may not, on occasion, serve as a temporary or short-term investment vehicle for those who seek to market time funds offered by other investment companies.

The following discussion regarding U.S. federal income taxes is based upon laws in effect as of the date of this prospectus and summarizes only some of the important U.S. federal income tax considerations affecting the Fund and its U.S. shareholders. This discussion is not intended as a substitute for careful tax planning. Please see the Fund’s SAI for additional U.S. federal income tax information.

Distributions from your Fund’s net investment income and net realized capital gains are taxable, whether you choose to receive them in cash or automatically reinvest them in additional Fund shares. The amount of taxes you owe will vary depending on your tax status and based on the amount and character of the Fund’s distributions to you and your tax rate.

Provided the Fund is able to maintain a constant NAV per share of $1.00, sales of the Fund’s shares generally will not result in taxable gain or loss. After the end of each year, the Fund will send to you a notice that tells you how much you have received in distributions during the year and their U.S. federal income tax status. You could also be subject to foreign, state and local taxes on such distributions.

In certain circumstances, you may be subject to backup withholding taxes on distributions to you from the Fund if you fail to provide the Fund with your correct social security number or other taxpayer identification number, or to make required certifications, or if you have been notified by the U.S. Internal Revenue Service (“IRS”) that you are subject to backup withholding.

A 3.8% Medicare tax is imposed on the net investment income (which includes, but is not limited to, interest, dividends and net gain from investments) of U.S. individuals with income exceeding $200,000, or $250,000 if married filing jointly, and of trusts and estates.

If you are neither a tax resident nor a citizen of the United States or if you are a foreign entity (other than a pass-through entity to the extent owned by U.S. persons), the Fund’s ordinary income dividends will generally be subject to a 30% U.S. withholding tax, unless a lower treaty rate applies. However, certain distributions reported by the Fund as capital gain dividends, interest-related dividends or short-term capital gain dividends and paid to a foreign shareholder may be eligible for an exemption from U.S. withholding tax.

and account holders or (b) if an intergovernmental agreement is entered into and implementing legislation is adopted, comply with the agreement and legislation. Other foreign entities will generally either need to provide detailed information identifying each substantial U.S. owner or certify there are no such owners.

Tax considerations for tax-exempt or foreign investors or those holding Fund shares through a tax-deferred account, such as a 401(k) plan or Individual Retirement Account, are generally different. For example, shareholders that are exempt from U.S. federal income tax, such as retirement plans that are qualified under Section 401 of the Internal Revenue Code generally are not subject to U.S. federal income tax on Fund dividends or distributions or on sales of Fund shares. Because each investor’s tax circumstances are unique and because tax laws are subject to change, you should consult your tax adviser about your investment.

The Fund does not have its own investment adviser. Instead, the Fund invests all of its assets in the Master Portfolio, which has an investment objective, strategies and policies substantially identical to those of the Fund. BFA serves as investment adviser to the Master Portfolio. The Master Portfolio may accept investments from other feeder funds. Certain actions involving other feeder funds, such as a substantial withdrawal, could affect the Master Portfolio and, therefore, the Fund.

Feeder funds, including the Fund, bear their respective master portfolio’s expenses in proportion to the amount of assets each invests in the master portfolio. The feeder fund can set its own transaction minimums, fund-specific expenses and conditions.

Under the master/feeder structure, the Board retains the right to withdraw the Fund’s assets from its Master Portfolio if it believes doing so is in the best interests of the Fund’s shareholders. If the Board decides to withdraw the Fund’s assets, it would then consider whether the Fund should hire its own investment adviser, invest in another master portfolio or take other action.

The Fund is a “feeder” fund that invests all of its assets in the Master Portfolio that has an investment objective, strategies and policies substantially identical to those of the Fund. BFA, a registered investment adviser, serves as investment adviser to the Master Portfolio. BFA manages the investment of the Master Portfolio’s assets and provides the Master Portfolio with investment guidance and policy direction in connection with daily portfolio management, subject to the supervision of MIP’s Board of Trustees. For its services to the Master Portfolio, BFA is entitled to receive a management fee at the annual rate of 0.10% of the Master Portfolio’s average daily net assets. BFA has contractually agreed to waive 0.03% of its management fee through April 30, 2021. BFA and BlackRock Advisors, LLC (“BAL”), the Fund’s administrator, have voluntarily agreed to waive a portion of their respective fees and/or reimburse operating expenses to enable the Fund and Master Portfolio to maintain minimum levels of daily net investment income. BFA and BAL may discontinue this waiver and/or reimbursement at any time without notice.

For the fiscal year ended December 31, 2019, BFA received a management fee, net of any applicable waivers, at an annual rate as a percentage of average daily net assets of the Master Portfolio as follows:

BFA is located at 400 Howard Street, San Francisco, California 94105. BFA is an indirect wholly-owned subsidiary of BlackRock. As of March 31, 2020, BFA and its affiliates had approximately $6.466 trillion in investment company and other portfolio assets under management.

A discussion regarding the basis for MIP’s Board of Trustees’ approval of the investment advisory agreement with BFA is available in the Fund’s semi-annual report for the period ended June 30, 2019.

From time to time, a manager, analyst, or other employee of BlackRock or its affiliates may express views regarding a particular asset class, company, security, industry, or market sector. The views expressed by any such person are the views of only that individual as of the time expressed and do not necessarily represent the views of BlackRock or any other person within the BlackRock organization. Any such views are subject to change at any time based upon market or other conditions and BlackRock disclaims any responsibility to update such views. These views may not be relied on as investment advice and, because investment decisions for the Master Portfolio are based on numerous factors, may not be relied on as an indication of trading intent on behalf of the Master Portfolio.

BAL provides the following services, among others, as the Fund’s administrator:

BAL is entitled to receive fees for these services at the annual rate of 0.07% of the average daily net assets of Capital Shares of the Fund. In addition to performing these services, BAL has agreed to bear all costs of operating the Fund, other than brokerage expenses, management fees, 12b-1 distribution or service fees, certain fees and expenses related to the Trust’s trustees who are not “interested persons” of the Fund or the Trust as defined in the 1940 Act and their counsel, auditing fees, litigation expenses, taxes or other extraordinary expenses. No additional administration fees are charged at the Master Portfolio level.

for the District of New Jersey against BlackRock Advisors, LLC, BlackRock Investment Management, LLC and BlackRock International Limited (collectively, the “Defendants”) under the caption In re BlackRock Mutual Funds Advisory Fee Litigation. In the lawsuit, which purports to be brought derivatively on behalf of Global Allocation and Equity Dividend, the plaintiffs allege that the Defendants violated Section 36(b) of the Investment Company Act by receiving allegedly excessive investment advisory fees from Global Allocation and Equity Dividend. On June 13, 2018, the court granted in part and denied in part the Defendants’ motion for summary judgment. On July 25, 2018, the plaintiffs served a pleading that supplemented the time period of their alleged damages to run through the date of trial. The lawsuit seeks, among other things, to recover on behalf of Global Allocation and Equity Dividend all allegedly excessive advisory fees received by the Defendants beginning twelve months preceding the start of the lawsuit with respect to each of Global Allocation and Equity Dividend and ending on the date of judgment, along with purported lost investment returns on those amounts, plus interest. The Defendants believe the claims in the lawsuit are without merit. The trial on the remaining issues was completed on August 29, 2018. On February 8, 2019, the court issued an order dismissing the claims in their entirety. On March 8, 2019, the plaintiffs provided notice that they are appealing both the February 8, 2019 post-trial order and the June 13, 2018 order partially granting Defendants’ motion for summary judgment.

The investment activities of BFA and its affiliates (including BlackRock, Inc. and its subsidiaries (collectively, the “Affiliates”)), The PNC Financial Services Group, Inc. (which, through a subsidiary, has a significant economic interest in BlackRock, Inc.) and its subsidiaries (each with The PNC Financial Services Group, Inc., an “Entity” and collectively, the “Entities”), and their respective directors, officers or employees, in the management of, or their interest in, their own accounts and other accounts they manage, may present conflicts of interest that could disadvantage the Fund and its shareholders.

BFA, its Affiliates and the Entities provide investment management services to other funds and discretionary managed accounts that may follow investment programs similar to that of the Fund. BFA, its Affiliates and the Entities are involved worldwide with a broad spectrum of financial services and asset management activities and may engage in the ordinary course of business in activities in which their interests or the interests of their clients may conflict with those of the Fund. BFA or one or more Affiliates or Entities act or may act as an investor, investment banker, research provider, investment manager, commodity pool operator, commodity trading advisor, financier, underwriter, adviser, market maker, trader, prime broker, lender, index provider, agent and/or principal, and have other direct and indirect interests in securities, currencies, commodities, derivatives and other instruments in which the Fund may directly or indirectly invest. Thus, it is likely that the Fund will have multiple business relationships with and will invest in, engage in transactions with, make voting decisions with respect to, or obtain services from, entities for which an Affiliate or an Entity performs or seeks to perform investment banking or other services. Specifically, the Fund may invest in securities of, or engage in other transactions with, companies with which an Affiliate or an Entity has developed or is trying to develop investment banking relationships or in which an Affiliate or an Entity has significant debt or equity investments or other interests. The Fund may also invest in issuances (such as structured notes) by entities for which an Affiliate or an Entity provides and is compensated for cash management services relating to the proceeds from the sale of such issuances. The Fund also may invest in securities of, or engage in other transactions with, companies for which an Affiliate or an Entity provides or may in the future provide research coverage. An Affiliate or Entity may have business relationships with, and purchase, or distribute or sell services or products from or to, distributors, consultants or others who recommend the Fund or who engage in transactions with or for the Fund, and may receive compensation for such services. The Fund may also make brokerage and other payments to Entities in connection with the Fund’s portfolio investment transactions. BFA or one or more Affiliates or Entities may engage in proprietary trading and advise accounts and funds that have investment objectives similar to those of the Fund and/or that engage in and compete for transactions in the same types of securities, currencies and other instruments as the Fund. This may include transactions in securities issued by other open-end and closed-end investment companies (which may include investment companies that are affiliated with the Fund and BFA, to the extent permitted under the Investment Company Act). The trading activities of BFA and these Affiliates or Entities are carried out without reference to positions held directly or indirectly by the Fund and may result in BFA or an Affiliate or an Entity having positions in certain securities that are senior or junior to, or have interests different from or adverse to, the securities that are owned by the Fund.

Neither BFA nor any Affiliate is under any obligation to share any investment opportunity, idea or strategy with the Fund. As a result, an Affiliate may compete with the Fund for appropriate investment opportunities. The results of the Fund’s investment activities, therefore, may differ from those of an Affiliate and of other accounts managed by an Affiliate, and it is possible that the Fund could sustain losses during periods in which one or more Affiliates and other accounts achieve profits on their trading for proprietary or other accounts. The opposite result is also possible.

more clients or BFA, its Affiliates or Entities or their directors, officers or employees, may have the effect of diluting or otherwise disadvantaging the values, prices or investment strategies of the Fund. The Fund’s activities may be limited because of regulatory restrictions applicable to BFA, one or more Affiliates or Entities and/or their internal policies designed to comply with such restrictions.

Under a securities lending program approved by the Board, the Trust, on behalf of the Fund, has retained BlackRock Institutional Trust Company, N.A., an Affiliate of BFA, to serve as the securities lending agent for the Fund to the extent that the Fund participates in the securities lending program. For these services, the securities lending agent will receive a fee from the Fund, including a fee based on the returns earned on the Fund’s investment of the cash received as collateral for the loaned securities. In addition, one or more Affiliates or Entities may be among the entities to which the Fund may lend its portfolio securities under the securities lending program.

The activities of BFA, its Affiliates and Entities and their respective directors, officers or employees, may give rise to other conflicts of interest that could disadvantage the Fund and its shareholders. BFA has adopted policies and procedures designed to address these potential conflicts of interest. See the SAI for further information.

The financial highlights tables are intended to help investors understand the financial performance of the Fund for the periods shown. Certain information reflects financial results for a single Fund share. The total returns in the tables represent the rate of return that an investor would have earned (or lost) on an investment in the Fund, assuming reinvestment of all dividends and/or distributions. The information has been audited by PricewaterhouseCoopers LLP, whose report along with the Fund’s audited financial statements, is included in the Fund’s combined annual report. You may obtain copies of the annual report, at no cost, by calling 1-888-204-3956 (toll-free) from 8:30 a.m. to 6:00 p.m. Eastern time on any Business Day.

There were no Capital Shares of the Fund outstanding as of December 31, 2019; as a result, the table below sets forth selected financial data for a SL Agency Share of the Fund outstanding throughout each year presented.

Electronic copies of most financial reports and prospectuses are available on BlackRock’s website at www.blackrock.com/prospectus/cash.

Shareholders Who Hold Accounts with Investment Advisers, Banks or Brokerages: Please contact your financial professional. Please note that not all investment advisers, banks or brokerages may offer this service.

The Fund delivers only one copy of shareholder documents, including prospectuses, shareholder reports and proxy statements, to shareholders with multiple accounts at the same address. This practice is known as “householding” and is intended to eliminate duplicate mailings and reduce expenses. Mailings of your shareholder documents may be householded indefinitely unless you instruct us otherwise. If you do not want the mailing of these documents to be combined with those for other members of your household, please contact the Fund’s Transfer Agent at (888) 204-3956.

The Fund is subject to the USA PATRIOT Act (the “Patriot Act”). The Patriot Act is intended to prevent the use of the U.S. financial system in furtherance of money laundering, terrorism or other illicit activities. Pursuant to requirements under the Patriot Act, the Fund is required to obtain sufficient information from shareholders to enable it to form a reasonable belief that it knows the true identity of its shareholders. This information will be used to verify the identity of investors or, in some cases, the status of Financial Intermediaries. Such information may be verified using third-party sources. This information will be used only for compliance with the requirements of the Patriot Act or other applicable laws, regulations and rules in connection with money laundering, terrorism or economic sanctions.

The Fund reserves the right to reject purchase orders from persons who have not submitted information sufficient to allow the Fund to verify their identity. The Fund also reserves the right to redeem any amounts in the Fund from persons whose identity it is unable to verify on a timely basis. It is the Fund’s policy to cooperate fully with appropriate regulators in any investigations conducted with respect to potential money laundering, terrorism or other illicit activities.

BlackRock is committed to maintaining the privacy of its current and former Fund investors and individual clients (collectively, “Clients”) and to safeguarding their non-public personal information. The following information is provided to help you understand what personal information BlackRock collects, how we protect that information and why in certain cases we share such information with select parties.

If you are located in a jurisdiction where specific laws, rules or regulations require BlackRock to provide you with additional or different privacy-related rights beyond what is set forth below, then BlackRock will comply with those specific laws, rules or regulations.

BlackRock obtains or verifies personal non-public information from and about you from different sources, including the following: (i) information we receive from you or, if applicable, your financial intermediary, on applications, forms or other documents; (ii) information about your transactions with us, our affiliates, or others; (iii) information we receive from a consumer reporting agency; and (iv) from visits to our website.

BlackRock does not sell or disclose to non-affiliated third parties any non-public personal information about its Clients, except as permitted by law, or as is necessary to respond to regulatory requests or to service Client accounts. These non-affiliated third parties are required to protect the confidentiality and security of this information and to use it only for its intended purpose.

We may share information with our affiliates to service your account or to provide you with information about other BlackRock products or services that may be of interest to you. In addition, BlackRock restricts access to non-public personal information about its Clients to those BlackRock employees with a legitimate business need for the information. BlackRock maintains physical, electronic and procedural safeguards that are designed to protect the non-public personal information of its Clients, including procedures relating to the proper storage and disposal of such information.

If you would like further information about the Fund, including how it invests, please see the SAI.

For a discussion of the Fund’s policies and procedures regarding the selective disclosure of its portfolio holdings, please see the SAI.

This glossary contains an explanation of some of the common terms used in this prospectus. For additional information about the Fund, please see the SAI.

Annual Class Operating Expenses — expenses that cover the costs of operating a share class of the Fund.

Daily Liquid Assets — include (i) cash; (ii) direct obligations of the U.S. Government; (iii) securities that will mature, as determined without reference to the maturity shortening provisions of Rule 2a-7 regarding interest rate readjustments, or are subject to a demand feature that is exercisable and payable within one business day; and (iv) amounts receivable and due unconditionally within one business day on pending sales of portfolio securities.

Distribution Fees — fees used to support the Fund’s marketing and distribution efforts, such as compensating financial professionals and other financial intermediaries, advertising and promotion.

Dollar-Weighted Average Life — the dollar-weighted average maturity of the Fund’s portfolio calculated without reference to the exceptions used for variable or floating rate securities regarding the use of the interest rate reset dates in lieu of the security’s actual maturity date. “Dollar-weighted” means the larger the dollar value of a debt security in the Fund, the more weight it gets in calculating this average.

Dollar-Weighted Average Maturity — the average maturity of the Fund is the average amount of time until the organizations that issued the debt securities in the Fund’s portfolio must pay off the principal amount of the debt. “Dollar-weighted” means the larger the dollar value of a debt security in the Fund, the more weight it gets in calculating this average. To calculate the dollar-weighted average maturity, the Fund may treat a variable or floating rate security as having a maturity equal to the time remaining to the security’s next interest rate reset date rather than the security’s actual maturity.

Other Expenses — include accounting, administration, transfer agency, custody, professional and registration fees.

Shareholder Servicing Fees — fees used to compensate securities dealers and other financial intermediaries for certain shareholder servicing activities.

Weekly Liquid Assets — include (i) cash; (ii) direct obligations of the U.S. Government; (iii) U.S. Government securities issued by a person controlled or supervised by and acting as an instrumentality of the U.S. Government pursuant to authority granted by the U.S. Congress, that are issued at a discount to the principal amount to be repaid at maturity without provision for the payment of interest and have a remaining maturity of 60 days or less; (iv) securities that will mature, as determined without reference to the maturity shortening provisions of Rule 2a-7 regarding interest rate readjustments, or are subject to a demand feature that is exercisable and payable within five business days; and (v) amounts receivable and due unconditionally within five business days on pending sales of portfolio securities.

THE FUND

BlackRock Funds III

BlackRock Cash Funds: Treasury

400 Howard Street

San Francisco, California 94105

1-888-204-3956

MANAGER

BlackRock Fund Advisors

400 Howard Street

San Francisco, California 94105

TRANSFER AGENT

State Street Bank and Trust Company

1 Heritage Drive

North Quincy, Massachusetts 02171

INDEPENDENT REGISTERED PUBLIC

ACCOUNTING FIRM

PricewaterhouseCoopers LLP

Two Commerce Square

2001 Market Street

Suite 1800

Philadelphia, Pennsylvania 19103

ACCOUNTING SERVICES PROVIDER

State Street Bank and Trust Company

One Lincoln Street

Boston, Massachusetts 02111

DISTRIBUTOR

BlackRock Investments, LLC

40 East 52nd Street

New York, New York 10022

CUSTODIAN

State Street Bank and Trust Company

One Lincoln Street

Boston, Massachusetts 02111

COUNSEL

Sidley Austin LLP

787 Seventh Avenue

New York, New York 10019-6018

This prospectus contains important information you should know before investing, including information about risks. Read it carefully and keep it for future reference. More information about BlackRock Funds III is available at no charge upon request. This information includes:

Annual/Semi-Annual Reports

These reports contain additional information about the Fund’s investments. The annual report describes the Fund’s performance, lists portfolio holdings, and discusses recent market conditions, economic trends and Fund investment strategies that significantly affected the Fund’s performance for the last fiscal year.

Statement of Additional Information

A Statement of Additional Information (“SAI”), dated April 29, 2020, has been filed with the Securities and Exchange Commission (the “SEC”). The SAI, which includes additional information about the Fund, may be obtained free of charge, along with the Fund’s annual and semi-annual reports, by calling 1-888-204-3956 on any business day. The SAI, as amended and/or supplemented from time to time, is incorporated by reference into this prospectus.

Questions

If you have any questions about the Fund, please:

Call:

1-888-204-3956 (toll-free)

8:30 a.m. to 6:00 p.m. (Eastern time)

on any business day.

World Wide Web

General fund information and specific fund performance, including SAI and annual/semi-annual reports, can be accessed free of charge at www.blackrock.com/cash. Mutual fund prospectuses and literature can also be requested via this website.

Written Correspondence

Write:

State Street Corporation

Institutional Transfer Agency

P.O. Box 5493

Boston, Massachusetts 02206

Overnight Mail

State Street Corporation

Institutional Transfer Agency

1 Heritage Drive

North Quincy, Massachusetts 02171

Portfolio Characteristics and Holdings

A description of the Fund’s policies and procedures related to disclosure of portfolio characteristics and holdings is available in the SAI.

For information about portfolio holdings and characteristics, BlackRock fund shareholders and prospective investors may call 1-888-204-3956.

Securities and Exchange Commission

You may also view and copy public information about the Fund, including the SAI, by visiting the EDGAR database on the SEC’s website (http://www.sec.gov). Copies of this information can be obtained, for a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov.

You should rely only on the information contained in this prospectus. No one is authorized to provide you with information that is different from information contained in this prospectus.

The SEC has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

INVESTMENT COMPANY ACT FILE # 811-07332

This Prospectus contains information you should know before investing, including information about risks. Please read it before you invest and keep it for future reference.

The Securities and Exchange Commission has not approved or disapproved these securities or passed upon the adequacy of this Prospectus. Any representation to the contrary is a criminal offense.

This table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of BlackRock Cash Funds: Treasury.

This Example is intended to help you compare the cost of investing in Institutional Shares of the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in Institutional Shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that Institutional Shares’ operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

U.S. Treasury obligations are backed by the full faith and credit of the U.S. Government. The principal and interest of all securities held by the Fund are payable in U.S. dollars.

The bar chart and table in this section provide some indication of the risks of investing in BlackRock Cash Funds: Treasury. The bar chart shows the returns of Institutional Shares of the Fund for each complete calendar year since the recommencement of operations of the Fund’s Institutional Shares. The average annual total return table compares the average annual total return of Institutional Shares of the Fund to that of the Money Fund Report (“MFR”) Treasury and Repo Institutional Average, a service of iMoneyNet. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. How the Fund performed in the past is not necessarily an indication of how it will perform in the future. If BFA and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. The Fund is a money market fund managed pursuant to the requirements of Rule 2a-7 under the 1940 Act. Updated information on the Fund’s performance can be obtained by visiting http://www.blackrock.com/cash or can be obtained by phone at 1-888-204-3956.

During the periods shown in the bar chart, the highest return for a quarter was 0.58% (quarter ended June 30, 2019) and the lowest return for a quarter was 0.12% (quarter ended March 31, 2017).

Current Yield: You may obtain the Fund’s current 7-day yield by calling 1-888-204-3956 (toll-free) or by visiting the Fund’s website at www.blackrock.com/cash.

Treasury Master Portfolio’s investment adviser is BlackRock Fund Advisors (previously defined as “BFA”).

The minimum initial investment for Institutional Shares of BlackRock Cash Funds: Treasury is $100 million, although the Fund may reduce or waive the minimum in some cases. You may generally purchase or redeem shares of the Fund each day the bond markets and the Fedwire Funds Service are open. To purchase or sell shares you should contact your financial professional or selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, Inc. and its affiliates) (each, a “Financial Intermediary”), or, if you hold your shares through the Fund, you should contact the Fund by phone at 1-888-204-3956, or by mail to State Street Bank and Trust Company, P.O. Box 5493, Boston, Massachusetts 02206.

Ask your individual financial professional or visit your Financial Intermediary’s website for more information.

The Fund is a government money market fund managed pursuant to Rule 2a-7 under the 1940 Act.

The Trust’s Board of Trustees (the “Board”) has chosen not to subject the Fund to liquidity fees or redemption gates due to declines in the Fund’s weekly liquid assets.

The investment objective for the Fund is to seek current income as is consistent with liquidity and stability of principal.

The following is a description of certain risks of investing in the Fund. The order of the below risk factors does not indicate the significance of any particular risk factor.

Income Risk — The Fund’s yield will vary as the short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates.

The Fund may also be subject to certain other non-principal risks associated with its investments and investment strategies, including:

The minimum initial investment amount for Institutional Shares of the Fund is $100 million; however, in certain situations, this minimum initial investment amount may be reduced or waived. Please contact your shareholder servicing agent (“Shareholder Servicing Agent”) or State Street Bank and Trust Company, the Fund’s transfer agent (the “Transfer Agent”), for more information.

Your Shareholder Servicing Agent may charge you a fee and may offer additional account services. Additionally, your Shareholder Servicing Agent may have procedures for placing orders for Institutional Shares that differ from those of the Fund, such as different investment minimums or earlier trading deadlines. Please contact your Shareholder Servicing Agent directly for more information and details.

Shares of the Fund are only registered for sale in the United States and certain of its territories. Consequently, the Fund generally does not accept investments from non-U.S. residents.

The Fund will open for business and begin accepting purchase orders at 8:30 a.m. (Eastern time) on any Business Day. Purchase orders for the Fund’s Institutional Shares must be received by 5:00 p.m. (Eastern time) on any Business Day.

Orders received after the applicable deadline for the Fund on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

Payment for Institutional Shares of the Fund may be made only in federal funds or other immediately available funds. You may be charged for any costs incurred, including any costs incurred to recompute the Fund’s NAV, in connection with a purchase order that has been placed but for which the Fund has not received full payment by the close of the federal funds wire (normally 6:00 p.m. Eastern time) on the day the purchase order was placed. This payment deadline may be extended by one Business Day where a purchase order is processed through certain

electronic platforms where same-day cash settlement is impracticable. The Fund will notify a shareholder or Financial Intermediary if its purchase order or payment was not received by an applicable deadline.

The Fund reserves the right to suspend or discontinue the offer and sale of its shares and reject or cancel any purchase order for any reason.

If you purchased shares through a Financial Intermediary that entity may have its own earlier deadlines for the receipt of the redemption order.

A direct buyer who has established an account with the Fund can add to or redeem from that account by phone or through the mail.

The Fund may postpone and/or suspend redemption and payment beyond one Business Day only as follows:

a. For any period during which there is a non-routine closure of the Federal Reserve wire system or applicable Federal Reserve Banks;

b. For any period (1) during which the NYSE is closed other than customary week-end and holiday closings or (2) during which trading on the NYSE is restricted;

c. For any period during which an emergency exists as a result of which (1) disposal of securities owned by the Fund is not reasonably practicable or (2) it is not reasonably practicable for the Fund to fairly determine the NAV of shares of the Fund;

d. For any period during which the SEC has, by rule or regulation, deemed that (1) trading shall be restricted or (2) an emergency exists;

f. For any period during which the Fund, as part of a necessary liquidation of the Fund, has properly postponed and/or suspended redemption of shares and payment in accordance with federal securities laws (as discussed below).

Under certain circumstances, if no activity occurs in an account within a time period specified by state law, a shareholder’s shares in the Fund may be transferred to that state.

Please contact your Financial Intermediary for details about payments it may receive from the Fund or from BFA, the Fund’s distributor or their affiliates. For more information, see the SAI.

The price you pay when you purchase or redeem the Fund’s shares is the NAV next determined after confirmation of your order.

The share price of Institutional Shares of the Fund is calculated by dividing the value of the net assets of the Fund attributable to Institutional Shares (i.e., the value of its total assets less total liabilities) by the total number of outstanding shares of such share class. The NAV per share is generally rounded to the nearest cent for the Fund.

The NAV of the Fund is determined on each Business Day as of 5:00 p.m. Eastern time (or, if the Fund closes early, at such closing time).

You begin earning distributions on your shares the day your purchase order takes effect. You continue earning daily distributions on your shares up to but not including the date you sell them.

distribution payment date. If you sell all shares before the monthly distribution payment date, the Fund remits all distributions accrued with the sale proceeds.

withholding, foreign financial institutions and investment funds will generally either need to (a) collect and report to the IRS detailed information identifying their U.S. accounts and U.S. account holders, comply with due diligence procedures for identifying U.S. accounts and withhold tax on certain payments made to noncomplying foreign entities and account holders or (b) if an intergovernmental agreement is entered into and implementing legislation is adopted, comply with the agreement and legislation. Other foreign entities will generally either need to provide detailed information identifying each substantial U.S. owner or certify there are no such owners.

BAL provides the following services, among others, as the Fund’s administrator:

BAL is entitled to receive fees for these services at the annual rate of 0.05% of the average daily net assets of Institutional Shares of the Fund. In addition to performing these services, BAL has agreed to bear all costs of operating the Fund, other than brokerage expenses, management fees, 12b-1 distribution or service fees, certain fees and expenses related to the Trust’s trustees who are not “interested persons” of the Fund or the Trust as defined in the 1940 Act and their counsel, auditing fees, litigation expenses, taxes or other extraordinary expenses. No additional administration fees are charged at the Master Portfolio level.

On May 27, 2014, certain investors in the BlackRock Global Allocation Fund, Inc. (“Global Allocation”) and the BlackRock Equity Dividend Fund (“Equity Dividend”) filed a consolidated complaint in the United States District Court for the District of New Jersey against BlackRock Advisors, LLC, BlackRock Investment Management, LLC and BlackRock International Limited (collectively, the “Defendants”) under the caption In re BlackRock Mutual Funds Advisory Fee Litigation. In the lawsuit, which purports to be brought derivatively on behalf of Global Allocation and Equity Dividend, the plaintiffs allege that the Defendants violated Section 36(b) of the Investment Company Act by receiving allegedly excessive investment advisory fees from Global Allocation and Equity Dividend. On June 13, 2018, the court granted in part and denied in part the Defendants’ motion for summary judgment. On July 25, 2018, the plaintiffs served a pleading that supplemented the time period of their alleged damages to run through the date of trial. The lawsuit seeks, among other things, to recover on behalf of Global Allocation and Equity Dividend all allegedly excessive advisory fees received by the Defendants beginning twelve months preceding the start of the lawsuit with respect to each of Global Allocation and Equity Dividend and ending on the date of judgment, along with purported lost investment returns on those amounts, plus interest. The Defendants believe the claims in the lawsuit are without merit. The trial on the remaining issues was completed on August 29, 2018. On February 8, 2019, the court issued an order dismissing the claims in their entirety. On March 8, 2019, the plaintiffs provided notice that they are appealing both the February 8, 2019 post-trial order and the June 13, 2018 order partially granting Defendants’ motion for summary judgment.

In addition, the Fund may, from time to time, enter into transactions in which BFA or an Affiliate or an Entity or their directors, officers or employees or other clients have an adverse interest. Furthermore, transactions undertaken by clients advised or managed by BFA, its Affiliates or Entities may adversely impact the Fund. Transactions by one or more clients or BFA, its Affiliates or Entities or their directors, officers or employees, may have the effect of diluting or otherwise disadvantaging the values, prices or investment strategies of the Fund. The Fund’s activities may be limited because of regulatory restrictions applicable to BFA, one or more Affiliates or Entities and/or their internal policies designed to comply with such restrictions.

Electronic copies of most financial reports and prospectuses are available on BlackRock’s website at www.blackrock.com/prospectus/cash.

If you would like further information about the Fund, including how it invests, please see the SAI.

For a discussion of the Fund’s policies and procedures regarding the selective disclosure of its portfolio holdings, please see the SAI.

This glossary contains an explanation of some of the common terms used in this prospectus. For additional information about the Fund, please see the SAI.

Annual Class Operating Expenses — expenses that cover the costs of operating a share class of the Fund.

Other Expenses — include accounting, administration, transfer agency, custody, professional and registration fees.

Shareholder Servicing Fees — fees used to compensate securities dealers and other financial intermediaries for certain shareholder servicing activities.

THE FUND

BlackRock Funds III

BlackRock Cash Funds: Treasury

400 Howard Street

San Francisco, California 94105

1-888-204-3956

MANAGER

BlackRock Fund Advisors

400 Howard Street

San Francisco, California 94105

TRANSFER AGENT

State Street Bank and Trust Company

1 Heritage Drive

North Quincy, Massachusetts 02171

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

PricewaterhouseCoopers LLP

Two Commerce Square

2001 Market Street

Suite 1800

Philadelphia, Pennsylvania 19103

ACCOUNTING SERVICES PROVIDER

State Street Bank and Trust Company

One Lincoln Street

Boston, Massachusetts 02111

DISTRIBUTOR

BlackRock Investments, LLC

40 East 52nd Street

New York, New York 10022

CUSTODIAN

State Street Bank and Trust Company

One Lincoln Street

Boston, Massachusetts 02111

COUNSEL

Sidley Austin LLP

787 Seventh Avenue

New York, New York 10019-6018

Annual/Semi-Annual Reports

Statement of Additional Information

Questions

If you have any questions about the Fund, please call: 1-888-204-3956 (toll-free) 8:30 a.m. to 6:00 p.m. (Eastern time) on any business day.

World Wide Web

Written Correspondence

State Street Corporation

Institutional Transfer Agency

P.O. Box 5493

Boston, Massachusetts 02206

Overnight Mail

State Street Corporation

Institutional Transfer Agency

1 Heritage Drive

North Quincy, Massachusetts 02171

Portfolio Characteristics and Holdings

A description of the Fund’s policies and procedures related to disclosure of portfolio characteristics and holdings is available in the SAI.

For information about portfolio holdings and characteristics, BlackRock fund shareholders and prospective investors may call 1-888-204-3956.

Securities and Exchange Commission

You should rely only on the information contained in this prospectus. No one is authorized to provide you with information that is different from information contained in this prospectus.

The SEC has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

INVESTMENT COMPANY ACT FILE # 811-07332

This Prospectus contains information you should know before investing, including information about risks. Please read it before you invest and keep it for future reference.

The Securities and Exchange Commission has not approved or disapproved these securities or passed upon the adequacy of this Prospectus. Any representation to the contrary is a criminal offense.

This table describes the fees and expenses that you may pay if you buy and hold Select Shares of BlackRock Cash Funds: Treasury.

This Example is intended to help you compare the cost of investing in Select Shares of the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in Select Shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that Select Shares’ operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

U.S. Treasury obligations are backed by the full faith and credit of the U.S. Government. The principal and interest of all securities held by the Fund are payable in U.S. dollars.

There are currently no Select Shares of BlackRock Cash Funds: Treasury outstanding as of the date of this prospectus. As a result, the chart, the table and the seven-day yield shown below give you a picture of the performance for SL Agency Shares of the Fund, which are not offered in this prospectus. The performance of the Fund’s Select Shares would be substantially similar to SL Agency Shares because Select Shares and SL Agency Shares are invested in the same portfolio of securities and performance would only differ to the extent that Select Shares and SL Agency Shares have different expenses. The actual returns and seven-day yields of Select Shares would have been lower than those of the SL Agency Shares because Select Shares have higher expenses than the SL Agency Shares.

Current Yield: You may obtain the Fund’s current 7-day yield by calling 1-888-204-3956 (toll-free) or by visiting the Fund’s website at www.blackrock.com/cash.

Treasury Master Portfolio’s investment adviser is BlackRock Fund Advisors (previously defined as “BFA”).

The minimum initial investment for Select Shares of BlackRock Cash Funds: Treasury is $1 million, although the Fund may reduce or waive the minimum in some cases. You may generally purchase or redeem shares of the Fund each day the bond markets and the Fedwire Funds Service are open. To purchase or sell shares you should contact your financial professional or selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, Inc. and its affiliates) (each, a “Financial Intermediary”), or, if you hold your shares through the Fund, you should contact the Fund by phone at 1-888-204-3956, or by mail to State Street Bank and Trust Company, P.O. Box 5493, Boston, Massachusetts 02206.

To be eligible to purchase Select Shares of the Fund, you must invest through an employer-sponsored or individual retirement savings plan; invest the proceeds rolled over from such retirement savings plan into an individual retirement account; or maintain an account with State Street Bank and Trust Company, the Fund’s transfer agent (the “Transfer Agent”), or with a shareholder servicing agent (a “Shareholder Servicing Agent”). Please contact your Shareholder Servicing Agent or the Transfer Agent for more information.

You may purchase or sell shares without paying a sales charge. Your order to purchase or sell shares must be received in proper form, as determined by the Transfer Agent or a Financial Intermediary pursuant to an appropriate agreement, by 5:00 p.m. Eastern time on any day the Fund is open (a “Business Day”) (or, if the Fund closes early, by such closing time) to purchase or sell shares at that day’s net asset value. Orders received after 5:00 p.m. Eastern time on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

Ask your individual financial professional or visit your Financial Intermediary’s website for more information.

The Fund is a government money market fund managed pursuant to Rule 2a-7 under the 1940 Act.

The Trust’s Board of Trustees (the “Board”) has chosen not to subject the Fund to liquidity fees or redemption gates due to declines in the Fund’s weekly liquid assets.

The investment objective for the Fund is to seek current income as is consistent with liquidity and stability of principal.

The Master Portfolio may transfer uninvested cash balances into a single joint account at the Master Portfolio’s custodian bank, the daily aggregate balance of which will be invested in one or more repurchase agreements.

The following is a description of certain risks of investing in the Fund. The order of the below risk factors does not indicate the significance of any particular risk factor.

Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.

Income Risk — The Fund’s yield will vary as the short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates.

There also is the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Fund may lose both the investment opportunity for the assets it set aside to pay for the security and any gain in the security’s price.

The Fund may also be subject to certain other non-principal risks associated with its investments and investment strategies, including:

The minimum initial investment amount for Select Shares of the Fund is $1 million; however, in certain situations, this minimum initial investment amount may be reduced or waived. Please contact your Shareholder Servicing Agent or the Transfer Agent for more information.

Your Shareholder Servicing Agent may charge you a fee and may offer additional account services. Additionally, your Shareholder Servicing Agent may have procedures for placing orders for Select Shares that differ from those of the Fund, such as different investment minimums or earlier trading deadlines. Please contact your Shareholder Servicing Agent directly for more information and details.

Shares of the Fund are only registered for sale in the United States and certain of its territories. Consequently, the Fund generally does not accept investments from non-U.S. residents.

The Fund will open for business and begin accepting purchase orders at 8:30 a.m. (Eastern time) on any Business Day. Purchase orders for the Fund’s Select Shares must be received by 5:00 p.m. (Eastern time) on any Business Day.

Orders received after the applicable deadline for the Fund on any Business Day (or, if the Fund closes early, at such closing time) will be cancelled.

Payment for Select Shares of the Fund may be made only in federal funds or other immediately available funds. You may be charged for any costs incurred, including any costs incurred to recompute the Fund’s NAV, in connection with a purchase order that has been placed but for which the Fund has not received full payment by the close of the federal funds wire (normally 6:00 p.m. Eastern time) on the day the purchase order was placed. This payment deadline may be extended by one Business Day where a purchase order is processed through certain electronic platforms where same-day cash settlement is impracticable. The Fund will notify a shareholder or Financial Intermediary if its purchase order or payment was not received by an applicable deadline.