Are the 3 chickens getting picked on different from the rest? different color, different breed? Chickens do not believe in equal rights and often pick on who is different. You may need to separate the different ones. Do you have enough space in your coop? Crowded birds pick on each other. Each bird needs at least 3 square feet of space. Add a head of cabbage, a pumpkin or squash for them to pick at, it may divert some activity or let them free range.
Any bird that is bloody needs to be separated as it will get picked until it has large, serious wounds.
To check if an egg is fertile crack one open and carefully slide out the yolk on a plate. In a strong light look for a small spot on the yolk. It will be white. In a fertilized egg this will lool like a white ring with a clear spot in the center. Unfertilized its a solid white spot. That means the rooster is doing his job. You can also incubate the eggs- they turn into chcks if they are fertile!
When you cross two breeds of chickens you get a mixed breed.