Could you get a picture of it? Gretchen often has "floss" in her stools from eating stuffing out of toys. Though it could still be worms, 3 de-wormings doesn't mean the dog is free of worms. Only stool samples can tell you if the dog is free of them. I had Gretchen dewormed 3 or 4 times with Drontal Plus and she STILL had worms. I ended up going to Tractor Supply and picking up Panacur and giving her a dose of that for 3 days straight and it took care of them. I used the horse dewormer (You'd need to get the OK from your vet and the dose for your dogs size) but they make packets of it already dosed out for dogs that can be mixed into food.