The flop hit you nicely with two overs and the nut flush draw: 3♥J♥6♣ The action is checked and you lead out for a little over half pot at $25. The SB flats and everyone else folds. Pot is $90.
The turn comes a sweet 9♥ and, to your surprise, the small blind leads out for $60. He has about $300 behind, and you have about the same. Facing a $60 bet and holding the nuts, what’s your plan?
Reads: PF, the SB flat-called what looks like a squeeze raise by you. He could have been doing this with any small or medium pair, big suited connectors, and things like suited Aces. His flat of your flop c-bet doesn't really narrow his range much, either, because your image is LAg, and he readily could assume you're c-betting flops like this. His donk on the turn is weird, but could easily be someone with a big two pair or better hand suddenly nervous about the third flush on board. He could also have a set or a non-nut flush or a big combo draw.
Estimate: you have the nuts, so you've got tons of equity. He's donking into you and has committed about a quarter of his stack, so he's getting pot committed.
Decide: You've got a big value hand, and there's a lot of money still to go into the middle. You are really only worried about a set turning into a full house.
Implement: Don't screw around and get FPS here by calling. You aren't building a pot by flatting. You also need to protect against the board pairing. Also, another heart on the board could easily kill anymore action. Just raise it up. A shove by you here could easily be interpreted as you just trying to take it down with a hand like A♥T♣ or similar. A villain holding even something as weak as two pair is likely to pay you off, and people with sets often find it impossible to laydown their hand, even if you give them the wrong odds on the river by shoving. Just get it in.
All-in for now...