two hens stressing other hen out

If it a group of 3 and 2 have ganged up on a single hen, it could be a very stressful situation for the 1 that is being picked on.  She may have stopped laying because she is moulting, but it is worth picking her up and checking her over.

If she is very thin, it could be she is being bullied off the feeders and you will need to separate her from the other 2.  You may even need to rehome her 

Isolation worked really well for us. When we got our two new girls our Bluebell was really nasty with them, constantly chasing them and pecking them quite viciously - generally terrorising them both.  We only isolated her for a couple of days and afterwards she was much better with them. Now they're all getting on fine. It is distressing when birds get bullied though. Ironically, it was the Bluebell that was bullied when she and another hen were first added to our previous pair. Chicken hierarchy!!