well actually you NEED 4 FEMALES. not three. three does nothing. four makes a pecking order.
NEVER keep males with females. people on here do it, but dont listen to all of them, its not good. maybe if you have a huge tank were they wont see eachother, but not in something so small like yours.
Finally, you need ATLEAST 10 GALLONS for four females. So, in your case you will only be able to keep one betta, no matter what sex, in that tank.
And if you want more fish, get some corydoras cat fish, or some otos for catfish, they need to be in groups of three plus. Or if you want something with a nicer color, get some micky mouse platties, which also should be in groups of three, 1m 2f.