Oh, ok. It sounds like you are doing everything just fine!
Maybe you should just feed all of them the better diet, so that the ones you keep for breeding turn out healthier. It would take longer for the rest of the broilers to grow out and they would probably be somewhat smaller, but it might...
I agree with QueenMisha. I also think you should keep more pullets, or one less cockerel. If you have three roosters for 10-12 hens, that is only 3-4 girls per rooster. That might work out, but it is pushing the limit. If you want to be able to have a better breeding program, I think it would be...