My three dogs dont get along?

we have a 5 year old, spayed female german shepherd (Lilly). we recently got two puppies. a male nuetered (Oswald) and a female spayed .(Rosie) we got the puppies when they were 10 weeks they are now 6 months.when one puppy is with the 5 year old they are fine but when the puppies are together with the old one they attack her.they try flipping her over and biting her. when one puppy rests the other atacks so the older one never gets a break. the older one tries snapping and growling but the puppies are relentless!