I have a 3 spot also in a 12g. He is very aggressive. He attacks everything in the tank. I was going to add another fish, but I know that the 3spot would rip it to pieces. My 3spot even attacks the algae scraper when I clean off the glass!! Its funny thought, he is also very territorial, he will pick up the hermit crabs any where in the tank and drop the in the left front corner. There is a pile of stuff there.
If you want to add another fish. I say bring back the 3spot and get something else.