If you really want to get into breeding and do it right (producing healthy fry), you MUST buy multiple males and females and then let them pair up. You can rescue hounded males, but only once it's obvious that they are not accepted. I have never needed to concern myself with identifying the gender in any apisto breeding setup... that's why I asked.
If you are trying to pick on male and two females and get into breeding, then good luck... you just rolled the genetic dice. Your customers might not want to breed the fry amongst themselves.
If you just bought three fish to enjoy, then that would be different.
So if breeding is your answer, then you need a minimum of three more fish and not a way to tell gender IMHO.