Not to discourage you from SW
, but a 3g tank is going to be very difficult to maintain. Water parameters will change so quickly in it that you'll have to keep a constant watch on them. It's too small for a clown. I can't think of any fish right now that would do well in that size tank. Perhaps a couple of shrimp...banded coral shrimp maybe? Possibly one of the smaller blennies or gobies.