Nut straight on monotone board, heavy action on the river

[Amy]

Live 20/40

I have KhTh on the button. Two decent but slightly loose guys limp from UTG and MP, and I raise. Blinds fold, both limpers call, three to the flop.

I didn't have any good reads on the villians, but MP seemed very straightforward, he'd flopped top two pair and a set earlier on fairly safe boards and didn't slowplay them in any way so his river check-3bet really confused me.

Flop: 3-J-Q, all diamonds.

Checked to me, I bet, UTG checkraises, MP cold calls, I call.

Turn: 2x

Checked to me, and I check.

River: 9x

Checked to me, I bet, UTG checkraises, MP 3bets [img]/images/graemlins/confused.gif[/img], I ??

Should I have 3bet the flop or bet the turn? Call this river? On the flop I'm fairly certain that UTG has top pair, but then when he checks the turn I'm not so sure anymore... I honestly have no idea what MP could have, there's a good chance that he would pound away on the flop if he has the flush, so I suppose I could hope for something like 9-9 or K-10?