<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0708</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0002018496</cik>
        <entityName>INVESTX SERIES (SPX-U3), A SERIES OF INVESTX MASTER LLC</entityName>
        <issuerAddress>
            <street1>19 Fulton Street</street1>
            <street2>Suite 300</street2>
            <city>New York</city>
            <stateOrCountry>NY</stateOrCountry>
            <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
            <zipCode>10038</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>212-390-9270</issuerPhoneNumber>
        <jurisdictionOfInc>DELAWARE</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Limited Liability Company</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2024</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>null</firstName>
                <lastName>InvestX Asset Management LLC</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>19 Fulton Street</street1>
                <street2>Suite 300</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10038</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Manager of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>null</firstName>
                <lastName>InvestX Markets LLC</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>19 Fulton Street</street1>
                <street2>Suite 300</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10038</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Promoter</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Affiliate of the Manager of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>null</firstName>
                <lastName>Great Lakes Fund Solutions, Inc.</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>500 Park Avenue</street1>
                <street2>Suite 114</street2>
                <city>Lake Villa</city>
                <stateOrCountry>IL</stateOrCountry>
                <stateOrCountryDescription>ILLINOIS</stateOrCountryDescription>
                <zipCode>60046</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Administrative Manager of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Robert</firstName>
                <lastName>McLamore</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Great Lakes Fund Solutions, Inc.</street1>
                <street2>500 Park Avenue, Suite 114</street2>
                <city>Lake Villa</city>
                <stateOrCountry>IL</stateOrCountry>
                <stateOrCountryDescription>ILLINOIS</stateOrCountryDescription>
                <zipCode>60046</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Officer of the Administrative Manager of the Issuer</relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Other Investment Fund</investmentFundType>
                <is40Act>false</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Decline to Disclose</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06c</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>0002018496-24-000001</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2024-03-19</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>false</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>40000</minimumInvestmentAccepted>
        <salesCompensationList>
            <recipient>
                <recipientName>Daniel Bernard Savoie</recipientName>
                <recipientCRDNumber>569391</recipientCRDNumber>
                <associatedBDName>InvestX Markets LLC</associatedBDName>
                <associatedBDCRDNumber>148248</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>19 Fulton Street</street1>
                    <street2>Suite 300</street2>
                    <city>New York</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10038</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>NC</state>
                    <description>NORTH CAROLINA</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Ashley Mofrad</recipientName>
                <recipientCRDNumber>7286124</recipientCRDNumber>
                <associatedBDName>InvestX Markets LLC</associatedBDName>
                <associatedBDCRDNumber>148248</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>19 Fulton Street</street1>
                    <street2>Suite 300</street2>
                    <city>New York</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10038</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>CA</state>
                    <description>CALIFORNIA</description>
                    <state>FL</state>
                    <description>FLORIDA</description>
                    <state>ID</state>
                    <description>IDAHO</description>
                    <state>TX</state>
                    <description>TEXAS</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Namhee Park</recipientName>
                <recipientCRDNumber>6724895</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>110 East Wilshire Avenue</street1>
                    <street2>Suite 301</street2>
                    <city>Fullerton</city>
                    <stateOrCountry>CA</stateOrCountry>
                    <stateOrCountryDescription>CALIFORNIA</stateOrCountryDescription>
                    <zipCode>92832</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>UT</state>
                    <description>UTAH</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>John Jason Ferneau</recipientName>
                <recipientCRDNumber>6136562</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>975 Lincoln Street</street1>
                    <street2>Suite 204</street2>
                    <city>Denver</city>
                    <stateOrCountry>CO</stateOrCountry>
                    <stateOrCountryDescription>COLORADO</stateOrCountryDescription>
                    <zipCode>80203</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>AZ</state>
                    <description>ARIZONA</description>
                    <state>CA</state>
                    <description>CALIFORNIA</description>
                    <state>CO</state>
                    <description>COLORADO</description>
                    <state>NE</state>
                    <description>NEBRASKA</description>
                    <state>WA</state>
                    <description>WASHINGTON</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Han Gu</recipientName>
                <recipientCRDNumber>6863727</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>2 Overhill Road</street1>
                    <street2>Suite 400</street2>
                    <city>Scarsdale</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10583</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>MI</state>
                    <description>MICHIGAN</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Rhett Price Grimes</recipientName>
                <recipientCRDNumber>6283241</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>11440 West Bernardo Court</street1>
                    <street2>Suite 300</street2>
                    <city>San Diego</city>
                    <stateOrCountry>CA</stateOrCountry>
                    <stateOrCountryDescription>CALIFORNIA</stateOrCountryDescription>
                    <zipCode>92127</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>NC</state>
                    <description>NORTH CAROLINA</description>
                    <state>SC</state>
                    <description>SOUTH CAROLINA</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Morris Gordon Nutt</recipientName>
                <recipientCRDNumber>2891940</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>1730 South Germantown Road</street1>
                    <street2>Suite 224</street2>
                    <city>Germantown</city>
                    <stateOrCountry>TN</stateOrCountry>
                    <stateOrCountryDescription>TENNESSEE</stateOrCountryDescription>
                    <zipCode>38138</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>TN</state>
                    <description>TENNESSEE</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Gary Glenn Garcia II</recipientName>
                <recipientCRDNumber>5027185</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>106 South Tampania Avenue</street1>
                    <street2>Suite 202</street2>
                    <city>Tampa</city>
                    <stateOrCountry>FL</stateOrCountry>
                    <stateOrCountryDescription>FLORIDA</stateOrCountryDescription>
                    <zipCode>33609</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>FL</state>
                    <description>FLORIDA</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Daniel M. King</recipientName>
                <recipientCRDNumber>5954543</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>11440 West Bernardo Court</street1>
                    <street2>Suite 300</street2>
                    <city>San Diego</city>
                    <stateOrCountry>CA</stateOrCountry>
                    <stateOrCountryDescription>CALIFORNIA</stateOrCountryDescription>
                    <zipCode>92127</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>CA</state>
                    <description>CALIFORNIA</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Judson Harris Gee</recipientName>
                <recipientCRDNumber>2453009</recipientCRDNumber>
                <associatedBDName>Kingswood Capital Partners, LLC</associatedBDName>
                <associatedBDCRDNumber>288898</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>2616 Fort Street</street1>
                    <city>Charlotte</city>
                    <stateOrCountry>NC</stateOrCountry>
                    <stateOrCountryDescription>NORTH CAROLINA</stateOrCountryDescription>
                    <zipCode>28205</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>NC</state>
                    <description>NORTH CAROLINA</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Aaron Wall</recipientName>
                <recipientCRDNumber>7805948</recipientCRDNumber>
                <associatedBDName>Fidelis Capital</associatedBDName>
                <associatedBDCRDNumber>322287</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>7200 Wisconsin Avenue</street1>
                    <city>Bethesda</city>
                    <stateOrCountry>MD</stateOrCountry>
                    <stateOrCountryDescription>MARYLAND</stateOrCountryDescription>
                    <zipCode>20814</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <state>MD</state>
                    <description>MARYLAND</description>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
        </salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>Indefinite</totalOfferingAmount>
            <totalAmountSold>3085000</totalAmountSold>
            <totalRemaining>Indefinite</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>34</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>138825</dollarAmount>
                <isEstimate>true</isEstimate>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse>The Issuer may require investors brought by a registered broker-dealer to pay up to 4.5% of the investor's commitment to such dealer-broker. In addition, selling agents and referring firms may receive a carried interest of 5.0%.</clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>308500</dollarAmount>
                <isEstimate>true</isEstimate>
            </grossProceedsUsed>
            <clarificationOfResponse>Represents a 3.0% one-time upfront management fee plus a 5.0% setup fee for organizational and operating expenses plus a 2.0% fee for assisting with setup of brokers and other selling participants, and other processing and documentation work.</clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>INVESTX SERIES (SPX-U3), A SERIES OF INVESTX MASTER LLC</issuerName>
                <signatureName>/s/ Marcus A. New</signatureName>
                <nameOfSigner>Marcus A. New</nameOfSigner>
                <signatureTitle>Officer</signatureTitle>
                <signatureDate>2024-04-15</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
