Breeding season is just around the corner and it seems your males has not yet determined who is alpha male in the pen.I had the same problem last year with my IB pen,,9 hens with a straight IB male,IB Pied male and an IB B/S male,,all of these birds grew up together and was penned together there entire life along with other birds.But last spring these 3 males were all put together to breed straight IB hens.Although they had an established pecking order in their previous much larger group,once seperated it was obvious the IB Pied male,along with the straight IB male were at the top of the food chain.Each evening the IB B/S male was being chased or pursued by one of the other two males.They were in the largest pen so they always had room to get away,ect,,occassionally I would intervene in the evenings at feeding time but as soon as I left the pen,the chase was back on.
If it's impossible to seperate some of the males,you may need to put some "cover" in the pen for the ones being dominated over has a way or place to escape. Using 1/2 of a large dog shipping crate works great,or something that will allow the bird to at least get his head away from the offender.Their favorite place to peck is the top of the head or neck area.
Maybe divide your large pen into 3 smaller ones using 1 male in each pen with a hen? One 3yr old male can cover 3 hens easily,,maybe sell the other 2 males? The fighting will continue until breeding season is over and thats normally until August,so you will need to do something.