3 people win a betting jackpot under one persons SSN. Federal taxes were deducted and remaining $ was split equally. How is this properly reported between the 3 people?

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.

The person who claims the ticket and pays it to the others will fill out form 5754 and turn it in to the lotto commission or other payer of the prize when the prize is claimed. The payer will use it to issue W-2G forms to each winner.  If it wasn't split up front then you can still claim only your portion and attach a statement to the return showing the information and allocations to each person. It's better to split it when the ticket is originally cashed as you are relying on the other members of your group claiming their portion otherwise you can find yourself in a mess with the IRS. It's probably best to seek advise from a professional.