I have 3 cats that are all being treated for fleas they are losing fur and weight how can i help them?

For 3 months I have been treating my 3 indoor only cats with frontline plus. I also used a premises spray made by VetKem. They are still scratching alot. I know the fleas caused them to scratch and chew sores all over and they are almost bald because of it. They are all eating and drinking well but they have all lost a good bit of weight. I don't know what else I can do to help them. I am so afraid of loosing them. Please help my Snuggles, Bounce and Downey.