You could either separate the drakes or get a couple more females.
I'm not an expert, so I hope someone else will jump in here and give some specific advise, but Bumblefoot is an infection that starts with a scratch or nick in the skin that lets bacteria in. It can get very serious, so if you need help treating it, I would take the duck to the vet. I remember when I was a kid, my sister's pet duck had bumblefoot. She lived in our spare bathroom tub for awhile. There was a lot of cleaning and wrapping the foot and antibiotics involved. Eventually the duck died.
Changing the diet won't heal bumblefoot, but if their diet is lacking in some way, it will make all your ducks healthier to get them on a better diet. Also, make sure that your ducks aren't walking on any sort of rough or sharp surface. That will cause injuries that let the bacteria in to cause bumblefoot.