Flop is WA/WB situation so you should really only be betting if worse hands will call. A worse Ace will call a flop bet so you should bet the highest amount you think AJ/AT will call. Is that amount $3? I don't think it is. Your flop bet puts you in a kinda tricky spot because you look a little weak. MP2 could be making a move (although this is probably not too likely at 25NL on Stars), but he probably has you beat. Checkraise in a multiway pot on a really really dry board = be very afraid. Villain is definitely not bluffing on the turn so he either has a set or AK or AQ (which he usually doesn't). I don't think AJ/AT make this turn bet. So it's really between a set and AK. Given his flop line I think a set is most likely here.