Are the plants in the bettas tanks silk, real or plastic? If they are plastic it is possible that they might be tearing their fins. If so remove the plastic plants ans see what happens, if there is no further damage then they were just tearing their fins and removing the plastic plants will solve the problem.
If you do not have plastic plants or even if you do but that the damage continues to get worse even after removing them then it is possible that they may have fin rot. This is usually caused from not cleaning their tanks enough. What size are their tanks and how often do you clean them? always be sure to remove any waste from the gravel when you do the cleaning.
If fin rot is the cause up their water changes and use a medication such as triple sulfa or Mardels Maracyn two. I hope they are all healthy for you again soon
Let me know how they go!