Both players cast Comeuppance. After resolve one of them cast Volcanic Hammer targeting his opponent.
Who will get this 3 damage? Is the result of a game is a draw?

Nobody will take the damage, because the game will end in a draw.

In this situation, the game enters an infinite loop of each Comeuppance preventing damage and trying to deal it back, only for the other one to send it right back. Since this damage is a new event, the first Comeuppance applies again and starts the whole process over–lather, rinse and repeat ad nauseam. There's no escape from the loop, so the game's a draw.

Edited Callum Milne (June 24, 2015 09:39:40 AM)

Posting to point out a quick edit–the answer I gave was correct, but I messed up the reasoning.

When a prevention effect includes an additional effect, such as the damage Comeuppance deals, the prevention takes place at the time the damage would have occurred, and the additional effect occurs immediately after, making it a new event. This means that the damage sent back and forth between the two Comeuppances is an endless stream of fresh, new events, each of which is in turn prevented to create the next link in the chain.