[Harv72b]

Just because they're LAGs doesn't mean they're stupid.

Based on UTG's 50/30 stats, we can narrow down his range of hands pretty well--he does not have a pocket pair, and he does not have the case ace, as he would've raised with either of those holdings. That leaves three possibilities: he's got a gutshot draw with 23/34/24, he's calling down with two unimproved cards in hopes that he can win by pairing the river/kickers, or he decided to slowplay the flop with a 5 and then realized that the turn card counterfeited him. He'll probably call down with a 5 (most players would), but I can't imagine him calling a raise on the turn with any of his other possible holdings.

SB has a history of leading the flop & turn and then check/folding the river--basically, he loves to bluff, but has the sense to figure out that whatever it is he was betting is not good when someone calls his bets on two streets, and then bets when checked to on the river. But, he's also passive preflop, so it is possible that he's holding a pocket pair or the case ace. With a pocket pair he might decide to call down if I raise the turn, or he might decide to fold immediately and believe that either I or UTG has him beat. If he's got the case ace, he 3-bets my turn raise and blows UTG out of the hand, and I end up chopping a bigger pot with him (and, incidentally, I can count on him to lead the river, allowing me to raise there & hopefully get another bet out of UTG first).

That was my thinking--there simply weren't very many possible hands that were worth calling down with on this board, and that list gets even smaller when someone raises the double-paired turn, especially when that person raised preflop. So my primary interest became getting that extra bet out of UTG on the river, instead of folding them both out with a turn raise when I'm holding the near-nuts (and the absolute worst case scenario had 4 cards left in the deck which could hurt me).

As far as flat calling the flop, I've been re-reading TOP and this seemed like a classic way ahead/way behind situation, even though it's 3-handed rather than HU. If I raise & I'm behind, then I probably end up with the third guy getting blown out of the pot & being forced to call down with a losing hand that has little chance of improving to a winner. If I'm ahead, then I run the chance of scaring both opponents out of the pot on the cheap street. FWIW, I had planned to raise the turn if I saw the same bet/call action leading up to me, but changed my mind for the above reasons when the turn filled me up.