Jam the flop.

I'm not sure about betting out on the river - MP1 may be typically loose post-flop, but he calls two BBs cold on the turn. He may have 9T, but I think there's a bigger chance the river completed his flush. If he does have a flush, you may be facing two bets cold when it gets back to you on the river. MP3 will bet for you on the river if you're worried about missing out on value. In fact, maybe you should check-raise: after, MP1 checks, it is less likely that he has a flush. I'm pretty sure you have MP3 beaten (AJ, KJ, J8, 55 are likely hands for him).