Unless these are fry, you should be able to tell if its a female or not. They gain the orange splash on their stomach very very early.
My three, 2 males and 1 female, currently coexist in a 90 gallon with three 6" oscars. (One is gonna leave soon). No fighting currently occurs between the trio of convicts, though the two smaller oscars are currently picking on the larger (Hence, I got to get rid of him)