Help! Drakes picking on duck hen

[Callduckies]

My two drakes have been picking on one of my 3 hens. They aren't overmating her but it does look like the feathers on the side of her chest are getting pulled on. They run at her when she gets near and peck at her. I will be getting more ducks to even out my ratio and I ordered a crate for drake jail. Will that help to ease the aggressive behavior? Is there anything else I can do?

 

[Callduckies]

They have no problems with my other two hens!