I don't think I've ever folded a set on the flop. There are some rare situations when you're up against a nit making a huge raise where you might be able to lay one down, but if you never fold them you'll be fine. On 3-flush or 3-straight boards, just play sets as fast as possible. Bet close to pot, push over raises, etc. If you're ahead they'll think you're drawing and call with worse, if you're behind you have outs.