Revolution is my first choice when it comes to cats and parasite prevention and/or treatment, but, I agree, always check with your veterinarian first.
Revolution is a topical prescription product available through veterinarians for the treatment of the adult, egg, and larvae (off label) stages of the flea, roundworms, hookworms, and ear mites, and the prevention of heartworms. Some veterinarians will ask you to re-apply Revolution as early as 2 weeks depending on the severity of the infestation. Application for preventative measures is once every month.
Roundworms do cause loose stool among various other symptoms which should subside as the worms leave the cat through the stool. Repeating the worming 2-3 times is important because the worming medication will not have an effect on the immature adult and migrating larvae stages of the roundworms. Only when the worms become adults can the be killed. There are many more "not-so-pleasant" aspects of the roundworm, but I'll just keep it simple.
You also want to make sure that you scoop the litter box at least once daily (preferably more often) and/or remove all feces from the environment because re-infestation is very easy. Make sure to wash your hands frequently because transmission of roundworms from pet to humans is possible and roundworm eggs in the right environment can remain infective for years to both you and your cats.
Be sure to have your veterinarian check a stool sample for any other intestinal parasites that are not visible to the eye and to make sure the worming agent is remaining effective. Revolution is designed to effectively treat roundworms but may require 2 or even 3 applications to get the situation under control. Just a recommendation, but I would continue applying Revolution monthly even after the symptoms subside to prevent future re-infestations. Some situations may require additional oral medication.
I hope I have been of some help to you and anyone else having similar problems!