definately bet that flop. your set of kings isnt going to look so good anymore if a fourth heart falls on the turn.

Also you get more information by betting the flop. If you get c/raised, probably smooth call, and look to spike a board pair. If you 3 bet into a flopped flush, the pot is going to get out of control fast, and is going to be hard to get away from. Also, if you 3 bet and he re-reraises its probably going to be a "lets play for stacks" raise. If he pushes, for example, you probably have to find a fold (assuming full stacks?), and thus you lose value from any board pairs that might be coming on the turn or river.