Mixing O's with any fish small enough for them to swallow almost always ends in expensive meals (notice I said almost). Firemouth's can definitely work but make sure to either grow them out a bit or buy larger specimens; the O's will definitely outgrow them if they are added all at once. There are lots of other choices as well as far as possible fish to add. Pretty much anything that doesnt get too massive and doesn't stay small ( over 4 or 5" ) and can thrive in the same water parameters is a possible choice; just make sure to avoid known "mean" fish, as Oscars are just big pussy cats and will get beat up by anything with a real attitude.
As far as having 3 Oscars in the same tank, well that's just a coin toss really. Be prepared to possibly re home one of them if they don't get along. I have had 3 in the same tank before but from some of the stories I've read, I'd consider myself very lucky that they all got along.
