How do I stop an aggressive rooster from beating up our duck?

We got two day old chicks and a day old duckling 2 weeks before Easter. They are now several months old, and the results are a hen, a rooster, and a drake [male duck]. We know hes a male because hes almost got all of his adult green and red mallard feathers. The hen and the duck spend almost all of their time together, and whenever rooster and hen would fight the duck would come in between them and break it up. Well recently the rooster has started ignoring the hen, but now he focuses his attention on our duck. Now rooster, who is bigger than the duck chases the duck and the hen has to get in between the two to stop the roosters aggression. They are not food animals, they are strictly pets and they currently live in a closed 8ft x8ft pen, with a large coup [with nestboxes and roosts] on the side. They also have another 8ft x 8ft pen that is in the middle of a gry, bug filled area that we put them in for playtime after dinner. They all grew up together, and rooster has only shown aggression since we got back from an 2 day/overnight stay at a friends house. Is this just a phase or will they go back to getting along? They are all near ual maturity, if not already, and the duck was imprinted to think he is a chicken. Im guessing Ritz [the duck] thinks he too is a rooster but he is smaller than Elvis [the big meanie].