* grunch *

With all the betting preflop it is strange that no one decided to lead on the flop. I probably would have bet out to see where everyone was at, with the idea to call one bet but probably fold to two. If by chance you lose one of these guys before the turn, as well, that would dramatically improve your chances.

Anyways, back to the question at hand, on the turn you have to now check/fold. I have to think MP3 has a smaller big pair say QQ-TT, possibly AK, AQ, KQ suited. We can figure UTG+1 for more hands than your TAG, of course, but it is difficult to narrow him down without a bet on the flop. All of this popped into PokerStove you about 16% equity on the turn so about a $1. You are out of position and have to fear a raise so check and then lay it down to a bet.