He'd have to be a total moron to slow play trips or better on that turn.
I guess if he is worried about a c/r he could have AJ/AQ here. AK probably bets turn.

This is a good place to bluff raise, since you "obv." don't have anything... floating with 33, but doesn't like the turn A?

I agree with Phant. that a 9 is possible... but how does the 3 give him 2 pair (that he doesn't already have)

Wait, I just realized that the flop $2 is 1/12 of the pot min/bet. That means he has like monster/not much. I can't see why he would check behind the turn with a monster.

He probably has Quads and wants to show them.