Sec Form 3 Filing - ECP Cardinal Holdings, LP @ Custom Truck One Source, Inc. - 2021-12-20

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person ECP Cardinal Holdings, LP 2. Issuer Name and Ticker or Trading Symbol Custom Truck One Source, Inc. [ CTOS] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 40 BEECHWOOD ROAD 3. Date of Earliest Transaction (MM/DD/YY) 12/20/2021 (Street) SUMMIT, NJ07901 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 25,859,767 I See Footnotes( 1 )( 2 )( 3 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ECP Cardinal Holdings, LP 40 BEECHWOOD ROAD SUMMIT, NJ07901 X X Signatures ECP CARDINAL HOLDINGS, LP. By: NESCO Holdings GP, LLC, its general partner, By: /s/ Rahman D'Argenio, President 12/29/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Includes: 4,500,000 shares held of record by ECP Cardinal Holdings, LP, which were formerly held directly by Energy Capital Partners III, LP, Energy Capital Partners III-A, LP, Energy Capital Partners III-B, LP, Energy Capital Partners III-C, LP, and Energy Capital Partners III-D, LP. and contributed to ECP Cardinal Holdings, LP. Also includes 21,238,988 shares held of record by NESCO Holdings, LP and 107,148 shares of Common Stock and 13,631 restricted stock units beneficially owned by Energy Capital Partners Management, L.P. ( 2 )The general partner of each of ECP Cardinal Holdings, LP and NESCO Holdings, LP is NESCO Holdings GP, LLC. Energy Capital Partners III, LP, Energy Capital Partners III-A, LP, Energy Capital Partners III-B, LP, Energy Capital Partners III-C, LP, and Energy Capital Partners III-D, LP (together, the "ECP III Funds") and Energy Capital Partners III (NESCO Co-Invest), LP are the sole members of NESCO Holdings GP, LLC. Energy Capital Partners III, LLC is the general partner of (i) Energy Capital Partners GP III, LP, which is the general partner of each of the ECP III Funds, and (ii) Energy Capital Partners GP III Co-Investment (NESCO), LLC, which is the general partner of Energy Capital Partners III (NESCO Co-Invest), LP. As a result, each of the foregoing entities may be deemed to share beneficial ownership of the shares of Common Stock held by ECP Cardinal Holdings, LP and Nesco Holdings LP. ( 3 )(Continued) ECP Management GP, LLC is the general partner of Energy Capital Partners Management, LP and may be deemed to share beneficial ownership of the securities beneficially owned by Energy Capital Partners Management, LP. ECP ControlCo, LLC is the sole member of Energy Capital Partners III, LLC and the sole member of ECP Management GP, LLC and may be deemed to share beneficial ownership of the securities beneficially owned be each of Energy Capital Partners III, LLC and ECP Management GP, LLC. The managing members of ECP ControlCo, LLC are Douglas Kimmelman, Andrew Singer, Peter Labbat, Tyler Reeder and Rahman D'Argenio all of whom collectively share the power to vote and dispose of the securities beneficially owned by ECP ControlCo, LLC. Each of the foregoing individuals and entities disclaims beneficial ownership of the securities reported herein except to the extent of his or its pecuniary interest therein.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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