I wouldn't ever do a 100% unless it's super dirty, or there is a disease going through it.
With my 20 gallon tank I end up doing a 10%-20% water change every week. I have platys in my 20 gallon tank, so with three bettas I would go with a 10% water change every week, and remember to vacuum every few weeks. This is just the way I would do it. I am not the all knowing water change god XD You just need to find what is good for you, and how much the ammonia builds up.