What are you giving her? any vitamins for support. I'd even add some Brewers Or Nutritional yeast to her feed. 1 Tab to 1 cup of feed. It can't help and may help. be worth a try.
What about the drakes could they be over mating her?2 drakes to 3 females isn't a good ratio.
@WmMike has she started laying yet? and if so when was the last time if you know. She maybe egg bound, Can you bring her inside and put her in a tub of warm water and let her float for a while with you staying with her. ? If she has an egg that needs to come out sometimes floating in warm...