This is what Im putting in my 180 after I use it as a growout (wich is gonna be a while) Im going to use black-sand as substrate a few pieces of driftwood and the fish im going to put in are: One Jag, one red tiger motaguense, and one lioselli, maybe another large cichlid possibly an oscar,
But since you already have three fish, I would suggest lioselli or red tiger motaguense, just be carefull as they get a little agro sometimes.