If I got one super male and 3 females, I wonder if one female might turn into a sub male?

Sub males are usually pretty easy to get a hold of. More common than females. You might get two super males that will fight all the time for the remaining two females. In my trio my super male flashes lots at the female mostly. The sub male is mostly ignored. I like my trio, I don't think adding another female to the mix would do any good.