With 3, you may run into trouble... If you have 2m, 1 f, the 2 males could harrass the female. If you have 2f and 1m, the male and female could pair up and harass the other female. If you can tell that you have at least one male and one female, I *personally* would take the other fish back to the fish store or would be ready to at the first sign that there's something more serious going on than just squabbles.