Kittins vs human

When is my 3 month year old cats going to stop feeding on there mom and when is her milk going to dry up

If they are 3 months old, they should have already been pushed away by the mother cat. If she hasn't done this, then you need to separate them until her milk dries up. If they are not separated, the mother cat can become very weak from the kitten taking all of her nutrients.
Sam

Ditto to above, Mom will keep producing milk and feeding the kittens, I am suprised she has not pushed the little teething monsters away, most mothers start getting bite by the babies and push them away, you need to step in and feed the kittens and let mom dry up