Aggressive fish issues?

I started with a 15 gallon tank of three fish that were the remains of several tanks that I consolidated down into one... This tank had a female black molly, a male platy, and a female betta. The fish all got along fine for a while, but eventually the male platy began being quite aggressive and chased the molly non-stop... It was clearly stressing the molly out, so I passed the platy on to a friend and kept the tank with just the female molly and the female betta... two days after getting rid of the platy, the molly has begun chasing the betta non-stop! She is nipping at her and violently lunging at her... the betta's fins are starting to look a big ragged and I feel terrible for her!

Whats going on with my fish?! What should I do? Should I pass both fish on and swap to a different type of fish all together? Should I add something to calm them down? Should I keep one and not the other and add a different fish? :S

Thanks for your help.

Keeping fish in a large school will always help, therefore, the fish will pick on their own kind and ignore the others. for your tank, a small group of 5 mollies would do the trick. Also, providing lots of plants will provide hiding places for the Betta. You could easily fit a few more fish into your tank.
Don't worry, fin - nipping is normal for many fish, and your Betta's fins will grow back soon. Check for signs of stress, and separate them if necessary until you get a larger group of mollies.

Experienced aquarium Hobbyist

Betta fish can not be together they are aggresive fish.