Are we talking a 3' tank, personally I would not recommend any eels, even the smallest pebbletooths which will reach 24-30". A ribbon eel will reach the length of the tank.
If you wanted to keep it a pure predator tank you could comfortably do a combo of about 4 maybe 5 lions and scorps depending on the species. A couple of dwarf or medium bodied lions and couple of ground dwelling scorp like a barbfish, leaf fish, or cockatoo wasp. A rock dwelling scorp like a yellow spotted. You want to go big time, get a rhino. Check out some of my threads because they all require specialized care, click my name and "find all threads.
@Slocke how about engineer gobies