CanI use a flea shampoo on a cat thats been treated with Advantage?

I have 3 cats. they all have fleas. Ive taken them to the vet and they have been treated with Advantage. Im treating my house, carpets, washing clothes and bedding. Im still finding fleas on my cats when I brush them, which is about twice a day. Is it ok to use a flea shampoo on them as well?