I have a male and female betta together but I'm told they're freaks of nature that should have killed eachother long ago (Obviously I didn't know this at the time I bought them, I think they're sibs they were in the same tank back then too).
So.... I wouldn't recommend it unless you have a divider in the tank to keep them apart.
What you COULD do is pick up another 5G hex (or smaller but really the 5Gs cost just a little more then the 3s so what's the point?
) and keep a trio of females together in that.
The females can easily reside together but there's a few things you need to keep in mind:
They are aggressive (not as aggressive as the males but still aggressive) so you need to provide hiding places (caves or plants) and distinct terratories they can claim (so one cave or plant per fish).
To reduce fighting it's best to have 1 or 3 never 2 or the same two will always fight. If you have three it sort of spreads it out and they seem to chill out.
I hope that helps!
For your male I would recommend calmer more passive tank mates. I just got some aquatic dwarf frogs and they are ADORABLE and would make great tank mates for him.