Personally, my betta hated platys and mollys. It could be different for you though. Make sure they aren't small enough that the betta can eat them. To introduce them, I would take the betta out, rearrange the tank a bit, add the platys and mollys first, and then add the betta back in. Thats how I did it. All bettas might react differently with other fish, so you may have better results than I did. Good luck!