Chicken Pecking and Loosing Feathers

[Golden_Yolk48]

We have had our chickens for about a year now maybe a little longer. There are three chickens that we got fully grown from a friend that were all in the same flock. There has never been a problem with any of them except the largest one would always block the ladder for the other hens to get into the upper level of our coop at night. All of the sudden a few weeks ago one of our chickens started pecking the large ones' head. We don't know what caused this to happen. The smallest of our chickens was not harming the other two, and would not go after the large chicken. Now all of the sudden the biggest chicken that is getting pecked at, is getting pecked at by both of the chickens, loosing her feathers and not laying at all. She also doesnt really move around much in the coop (which we keep her upstairs and the other two downstairs), or outside when we let them out. We are trying to figure out why this has started to happen and why our other two chickens are pecking at her. We have considered getting rid of this bigger chicken because she has stopped laying eggs and the other two seem to now hate her. What is the problem here!!!! Anyone have any clues!?!?
Thanks!

 

[Happy Chooks]

Has anything else changed in the coop? Does the biggest one seem sick? Chickens will spot a weak chicken and can gang up on it and kill it.

 

[Golden_Yolk48]

Nothing has changed in the coop, and at first we thought that the chicken might be sick because she was not moving around or doing much, but now it seems that she actually wants to be let out and moves around outside.