Aggressive Hen

I have ISA brown 3 hens that all are 4 years old. Recently one of the hens has had a weird behaviour, she is on top of the pecking order. She is starting to randomly attack the other 2 and one of the hens combs started to bleed. I have never seen such behaviour from a chicken. They fight like roosters and it's scary I have to break up the fight. Now the other 2 hens are afraid of the aggressive hen they run away whenever she walks nearby. I'm afraid soon she'll end up pecking the others to death

You should start by taking care of the birds being attacked. Clean their combs well, remove any blood you can, and apply an anti-picking lotion to the cuts. If any look especially nasty you can apply a topical antibiotic as well, Neosporin being ideal.

The hen who is being aggressive should be separated from the flock, out of both visual and hearing range. A large dog crate or carrier is ideal, you can place it anywhere away from the flock - around the other side of the house, or in a basement or laundry room works well. She should remain here for a minimum of a week, or two for best results. After this time period you should do a hard reintroduction - go out into the coop midday, and simply drop her into the middle of the run. Don't use any introductory precautions and don't let her see the other chickens or vice versa until the day you put her back in the pen. With luck, this will reduce her status in the pecking order - over the course of a week or two the birds will often "forget" the removed hen(s), and when they are returned to the coop they will usually have reduced social status since they are the "newcomers" to the flock.

If this fails, you might look into purchasing pinless peepers. They have worked well curbing aggression in my nastiest hens and cocks, if other methods failed.

You might also consider looking at your flock's environment and coop. Stress and crowding are two common causes of aggression. Consider providing additional feeders or waterers spread out across the coop and run, to prevent squabbling over resources, and maybe introduce some distractions for the birds. Good distractions include things like a flake of straw for them to tear apart and scratch in, or something like leafy greens hung on a string so they can more actively work for their food.

I have separate the aggressive one from the others now, I will see if this will work hopefully it does, fingers crossed. Thankyou so much