Welcome to the forum, glad you joined.
Three weeks ago you got a flock of mixed age/breed chickens. For these three weeks one of them has been getting picked on while on the roost by two specific hens and when just walking around during the day by others. Three weeks sounds like a long time for the pecking order still being sorted, I think something else might be going on.
It could be helpful to know how many total chicken you have, how much room you have in the coop and in the run (preferably in feet or meters), do these "walking around peckings" occur in any specific place or time, when do they have access to both the coop and run, what the roosts look like (how long and do you have more than one), breeds involved (especially does she look different like funny feathers), and ages. Did they all come from the same flock? The more we know about your actual situation the more likely we will notice something that might be a problem. Photos can often help, you never know what we might see.
Lack of room is a common cause of behavioral problems, that's why we ask, but it doesn't sound like that's the only problem to me. Still, it might be a part. My first thought is that there is an age (or actually maturity) difference. Is the victim laying? Mature chickens outrank immature chickens in the pecking order and can be pretty brutal in enforcing those pecking order rights. Typically my pullets don't make their way into the pecking order until they start to lay, then they are accepted. Until then mine tend to form a sub-flock and avoid the adults.
Chickens are social animals, they like to be with other chickens. But more mature hens are likely to peck immature ones if they invade their personal space. Is your victim alone and just trying to socialize but getting pecked for her effort?
Does the victim look weird, maybe silkie feathers or a weird hairdo like a Polish? If they grow up together that's usually not a big deal but occasionally when you integrate a difference can attract attention. That's more likely if they are crowded but could happen anyway.
When the victim goes to roost, does she go to the bullies or do they move to her? I read what you said but I've seen a hen leave her normal roosting place to go after weaker chickens at the far end of the roost. It's not always the dominant chickens either. I think it's one that is insecure in her position in the pecking order and takes the opportunity to brutalize a weaker chicken to try to keep it down. For whatever reason they do that, my immature pullets often won't sleep on the main roosts until they start to lay.
Is the victim unhealthy or crippled? Sometimes the flock will try to run off a weak chicken. They can be quite vicious about it. It's a survival technique, either to get rid of a sick chicken before they all get sick or a weak chicken can attract predators to the flock area. Does she act normal and have you checked to see if she is injured?
So what can you do? That kind of depends on what is going on. If she is young she may need to be separated until she matures. If you can identify instigators it can be helpful to isolate them from the flock for a while. That gives her a chance to make friends with the others plus can change the flock dynamics. The instigators lose their place in the pecking order and have other things to worry about when you put them back instead of being a bully. I try to solve for the benefit of the entire flock. More than once I've permanently removed a victim or a bully to maintain flock peace.
The right answer could be just to let them work it out as long as she is not being physically injured. That's my normal approach. But I do not force them to be in close contact with the others in order to force them to fight it out. I give them as much room as I can and let them work it out on their schedule, not mine.
I wish you luck with it, it's not always easy. And once again,
to the forum!
Chicken society, the pecking order, is sort of like mean girls in high school. As long as they are not drawing blood I would let them work things out. Removing the two dominant birds for several days might help things.
