I own 5 balloon mollies. Yes, they CAN be aggressive. One of my males loves to harass other fish and picked on my angelfish really badly. He also loves to beat up my other male mollie, but that second male is the SWEETEST thing ever.
Mollies do love to be schooled together, and need to be with others around their size due to the aggressiveness of some. 3 is a small amount unless you have 1 male and 2 females. If you do, then you'll be good! If they're JUST for show (you don't want to breed them or anything) the 1 inch to 1 gallon rule fits well. Mollies usually get to be about 2-3 inches, so for those 3 mollies you'll need about a 7-10 gallon.
The betta most likely won't do well with the mollies in the end. Unless you're willing to graduate to a 20 gallon with about 5 mollies (like myself) and all of the mollies are very calm and not fin-nipping brats (like my one male LOL).
My suggestion? Keep them all! Buy a 10 gallon from the store, PetSmart sells them for $12. A filter from WalMart is $10. Seran wrap the top of that thing! Then put the mollies in there with a couple others (general rule for mollies is 2-3 females per male so the harassing gets shared and not focused on one). Then use the 5 gallon you have for your betta! You could even split the tank in half and get a second betta ;)
I have no idea about Chinese Algae Eaters... But if that's true, I'd get rid of him!! o.0