I have this problem more often with chickens but sometimes with ducks. I would try separating the Male from the two females. Put him in a little pet carrier with a small amount of food or water. You can also divide the living quarters but separation of the male out of visible sight is recommended. It may be a hierarchy/food issue. Allowing the two females to reset structure without the male would help with this. For me it has never taken longer than 5 days separation.
Only other situation I can think of is that the submissive female is sick. If this is the case it should be visibly noticable soon. Isolate your weak duck and tend to the wounds with warm water and vinegar. Use blu kote if you see infection on wounds. If the sickness is internal you can see a vet about it or look up some antibiotics and make a educated guess. I very rarely treat sick birds and when I do it is mainly external infections so I am not an expert on duck internal illnesses. Best of luck to you and I hope your duck feels better.