Are the other two at the bottom of the pecking order? As Eggcessive says, it looks a lot like feather loss, which is most likely from mites or feather picking (other chickens picking at their head / necks). If it was mites I would expect more of your girls to be affected, but feather picking will only happen to those at the bottom of the order - especially at roosting time, when they are fighting for perches.
If you can, go into the coop at roosting time and stand quietly in a corner (or find a way to peek in there if it's too small for you to get into - I can't tell what type of coop you have from your photos!)
It is quite instructive to see how your girls react at bedtime - I have one girl who is quiet, calm, doesn't hassle the others at all during the day. She sits on the terrace steps next to two or three others during the day, no problem at all. Come bedtime she patrols the entire length of the main roosting perch, defying anyone to come and roost next to her! She fights them all off - pecking heads and necks, and then burrowing into their body fluff to try and persaude them off the roost!