X0306 4 2022-02-16 1 0001709682 Custom Truck One Source, Inc. CTOS 0001404073 Blackstone Holdings III L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK NY 10154 0 0 0 1 Former 10% Owner 0001393818 Blackstone Inc 345 PARK AVENUE NEW YORK NY 10154 0 0 0 1 Former 10% Owner 0001404071 Blackstone Group Management L.L.C. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK NY 10154 0 0 0 1 Former 10% Owner 0001070844 SCHWARZMAN STEPHEN A C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK NY 10154 0 0 0 1 Former 10% Owner Common Stock 2022-02-16 4 S 0 382460 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 472446 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 45860 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 23457 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 455487 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 466472 8.07 D 0 I See Footnotes Common Stock 2022-02-16 4 S 0 2264 8.07 D 0 I See Footnotes This amount represents the price per share of common stock, par value $0.0001 per share ("Common Stock"), of Custom Truck One Source, Inc. (the "Issuer") sold in a transaction exempt from registration pursuant to Rule 144 promulgated under the Securities Act of 1933, as amended. Reflects shares of Common Stock directly held by Blackstone Energy Partners NQ L.P. ("BEP NQ"). Reflects shares of Common Stock directly held by BEP CTOS Holdings L.P. ("BEP CTOS"). Reflects shares of Common Stock directly held by Blackstone Energy Family Investment Partnership SMD L.P. ("BE SMD"). Reflects shares of Common Stock directly held by Blackstone Energy Family Investment Partnership NQ ESC L.P. ("BE NQ"). Reflects shares of Common Stock directly held by Blackstone Capital Partners VI-NQ L.P. ("BCP VI-NQ"). Reflects shares of Common Stock directly held by BCP CTOS Holdings L.P. ("BCP CTOS"). Reflects shares of Common Stock directly held by Blackstone Family Investment Partnership VI-NQ ESC L.P. ("BE FIP"). Blackstone Energy Management Associates NQ L.L.C. ("BEMA") is the general partner of BEP NQ and BEP CTOS. Blackstone EMA-NQ L.L.C. ("BEMA NQ") is the managing member of BEMA. BEP Side-by-Side GP NQ L.L.C. ("BEP GP") is the general partner of BE NQ. Blackstone Management Associates VI-NQ L.L.C. ("BMA NQ") is the general partner of BCP VI-NQ and BCP CTOS. BMA VI-NQ L.L.C. ("BMA VI-NQ") is the managing member of BMA NQ. Blackstone Family GP L.L.C. ("BS GP") is the general partner of BE SMD. BS GP is wholly-owned by Blackstone's senior managing directors and controlled by its founder, Stephen A. Schwarzman. BCP VI-NQ Side-by-Side GP L.L.C. ("BCP GP") is the general partner of BE FIP. Blackstone Holdings III L.P. ("Holdings III") is the sole member of each of BEMA NQ, BEP GP and BMA VI-NQ. The general partner of Holdings III is Blackstone Holdings III GP L.P. ("Holdings III GP"). The general partner of Holdings III GP is Blackstone Holdings III GP Management L.L.C ("Holdings III GP Management"). Blackstone Holdings II L.P. ("Holdings II") is the sole member of BCP GP. Blackstone Holdings I/II GP L.L.C. ("Holdings I/II GP") is the general partner of Holdings II. Blackstone Inc. ("Blackstone") is the sole member of Holdings III GP Management and Holdings I/II GP. The sole holder of the Series II preferred stock of Blackstone is Blackstone Group Management L.L.C. ("BS Group Management"). BS Group Management is wholly-owned by Blackstone's senior managing directors and controlled by its founder, Stephen A. Schwarzman. Information with respect to each of the Reporting Persons is given solely by such Reporting Person, and no Reporting Person has responsibility for the accuracy or completeness of information supplied by another Reporting Person. Due to the limitations of the electronic filing system certain Reporting Persons are filing a separate Form 4. Each of the Reporting Persons (other than to the extent it directly holds securities reported herein) disclaims beneficial ownership of the securities held by the other Reporting Persons, except to the extent of such Reporting Person's pecuniary interest therein, and, pursuant to Rule 16a-1(a)(4) under the Securities Exchange Act of 1934 (the "Exchange Act"), each of the Reporting Persons (other than to the extent it directly holds securities reported herein) states that the inclusion of these securities in this report shall not be deemed an admission of beneficial ownership of all of the securities reported herein for purposes of Section 16 of the Exchange Act or for any other purpose or that the Reporting Persons are subject to Section 16 of the Exchange Act. Form 3 of 3 BLACKSTONE HOLDINGS III L.P., By: Blackstone Holdings III GP L.P., its general partner, By: Blackstone Holdings III GP Management L.L.C., its general partner, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Senior Managing Director 2022-02-18 BLACKSTONE INC., By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Senior Managing Director 2022-02-18 BLACKSTONE GROUP MANAGEMENT L.L.C., By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Senior Managing Director 2022-02-18 STEPHEN A. SCHWARZMAN, /s/ Stephen A. Schwarzman 2022-02-18