[crunchy1]

I can understand calling the first 2BBs on the river. It's probably very, very close and, at the same time, also very possibly -EV. However...

There's a reasonable chance that MP2 is taking a shot on the river and ALSO that MP3 recognized this and is trying to push out better hands with something like AT or 99. The combination of these possibilities is probably more remote than I give it credit for. However, given that this is a possible scenario, and also given that there's no fear of a 3-bet (MP1 will be all-in, MP2 is not going to bluff 3-bet) I can understand a call.

That being said - when MP3 raises the river, Hero calls, MP1 does commit to showdown by going all-in, and MP2 can still 3-bet - it's pretty obvious that one-pair is not going to win in this pot.

I don't think that Hero's reaction to the initial river raise (whether it be calling OR folding) is costing Hero as much as some people seem to think. I think that calling a river 3-bet, in a protected pot, with one-pair is tossing away an extra big bet because AA looks pretty and the pot was big.