well bettas arent always meant to be community fish, but maybe your other fish are a tad agressive, and bettas tails and fins are very delicate and easy to rip.
do you have any pointy or sharp plants, decorations or objects in your tank? if you do, i suggest silk/fabric plants, they wont hurt your bettas fins. also, if you have some sort of cave, replace it with a flowerpot (well washed of course) on its side in the aquarium. the sides are smooth and wont grab, tug or pull at your bettas fins. good luck, and also watch to see if your betta shows fear near your other fish, maybe it flees when they come near? and also watch for any bullying behaviour in your other fish.