If your boys get along, you could get 6-8 more girls and have everyone together. I've heard that drakes that were raised together don't fight as much and that has been my experience so far.
You may have to give her up. It's usually very easy to find a home for a female. I don't really know what else you could do, ducks are flock animals so she would need at least one friend to hang out with, play with, forage with etc. I did think that maybe one of your drakes could live with her and you just keep a close eye on them to make sure she doesn't get over mated but you still have the other drake who needs a friend.
Good luck and I hope things work out!