I had been very active up to this point. Isoing limpers a few times, led into 2 opponents when turning a flush with 56c on a J2c3c. Both called turn and folded river. Had to show down one bluff when leading a 643K5 board on river with 58off.

UTG (solid tag) raises to 300 at 50/100 with 12k eff.

MP+2 and HJ both call roughly 10k a piece. Both loose passive calling stations post flop.

In sb with Ac7c I flat with 18k.

BB loose fish also doesn't fold any pairs until river calls with roughly 12k as well.

Flop 9c6c3s.

Checks to UTG who bets 1200 into 1500. He had not been continuing post flop without strong hands especially when called in multiple spots.
Mp and HJ folds.

I call in sb, BB calls. Pot 5100

Turn 10c.

I check, BB leads 1600, UTG thinks for nearly a min and calls. Pot is 8300.

Flat, Jam, Raise to X amount?

I think you have your answer here. I jam, given that it's only a pot size bet and at least one of them is gonna call you. Shove also may look more semi bluffy perhaps something like Ac9x.

UTG pause may have been thinking of a raise with TT, or just getting worried his overpair was not good. I think KcKx and QcQx, both just call quite easily.