[MrDannimal]

My first thought was bet/3-bet (before reading responses). CO cold-calls PF, then is never played back at. UTG is raised on the flop and calls the raise and the turn bet. He doesn't re-raise the turn A, so he doesn't have one (or he has teeny-tiny balls).

It's 3 times more likely that CO has a naked ace than a boat, if you're assuming he has an ace (If you're willing to give him A3 or A4, then any A is possible and there are 3 kickers that boat and 9 that don't). Yes, he can have 33 or 44 (or AA, god forbid), but an ace is more likely IMO.

If you bet and UTG folds, then he wasn't going to call CO's bet and your call/raise. If UTG calls and folds to your 3-bet, then you get 4 bets if it's not capped. Same as if UTG calls 2 bets if you just call CO's raise. If UTG calls one bet but not a raise, you only get 3 bets (and this is one case where you won't know until too late. If you 3-bet, you're going to almost certainly get 1+3 bets, and maybe 3+3).

I REALLY think that CO has a boat here infrequently. I also think that most people aren't going to instantly see 56 as a holding if you donk out the river. When you 3-bet, maybe. I just think that in the long run you gain more bets 3-betting than you would lose calling a cap.