Person B and C can simply report the gambling income without paperwork. Person A is going to have a problem because, if they report 1/3 the amount on the W-2G, the IRS will send a mis-match letter, and there is no way to deduct the other 2/3 as an expense if person A reported the entire amount.
I am not an attorney or enrolled agent, so this is only my amateur recommendation. Person A needs to write a letter describing the arrangement, and have it signed by all 3 people. The letter should describe that the betting was a joint arrangement, it should say that all three bettors acknowledge responsibility for 1/3 the amount and agree to report 1/3 the amount on their tax returns, and it should include the names, addresses, and social security numbers of all 3 bettors. All three bettors should sign the letter. Person A reports 1/3 the amount, rather than the whole amount of the W-2G. When the IRS sends its mismatch letter (probably a CP2000 notice), person A will write a letter back to the IRS explaining the arrangement and including a copy of the signed agreement.
If the amount is large, you may want to pay for professional advice, especially if you are person A.
