as always you want to switch it up to avoid predictability.
With a set I think you can choose to be either aggressive or slow-playing. You do take a chance, though, when you slow play that someone miraculously hits a flush, full or straight...the more players that are in the pot with you and the more conducive a flop is to straight/flush/full draws, the bigger the risk is!
When it comes to trips hit on the flop, however, in most cases I like to come in pretty strong.
Why?
Because I don't want anyone to have a "free" or "nearly free" chance at hitting that full, straight, or flush. And if someone does happen to hit a flush/straight/full draw on the flop, you want them to have to pay dearly to see those next two streets. Many players won't have the stomach to risk a large part of their chipstacks on a draw possibility.
Why is it good for you that they pay to see those next two streets when you hit trips on the flop?
because when they fold, you bank chips with limited risk. When they call your bets on the flop, turn, and river and do not hit their draws you will have taken a great number of their chips. When they do hit their draws, on the turn or river at least you can say you put in your best effort to stop them from doing so too easily (i.e. you forced them to risk their chip stacks to see if they would hit their draws instead of limping along letting them see for free).
I think it is dangerous to slow play trips hit on the flop in many if not most situations. But you must also consider what the value of your set is and what the flop is: if you have trips of 2's through 5's, there is probably less risk of opponents having a fullhouse/straight draw (this logic follows organically from the fact that most players are playing high valued cards and folding, preflop, low valued cards...but this logic also assumes you bet something preflop because players will limp in with practically anything). If you are at a table with players who will put all their chips in teh middle with virtually anything, then you should probably be even more aggressive in your betting on the flop (perhaps even push all-in).
That's how I tend to play sets and trips, hit on the flop.