How can I get my new chicken to roost with the others?

Hello there

We had 3 chickens which were very happy together, but unfortunately we lost one of the chickens (the dominant one) so we replaced her with an 18 week chicken.  When we got her I sneaked her in the house when the others were asleep hoping they would all settle down together but unfortunately one of the chickens is really bullying her.

In their run there is alot of space for her to get away from the others and I have put out extra food and water but at night she does not want to go into the house to roost with the other 2 (understandably) so she flaps up into a tree or  sits on the fence.  I have been putting her in when the others are settled and getting up really early to let her out  but can't keep this up for too long.  We have only had her a week is this likely to settle down or should I think about finding another house for her to roost in?  Any suggestions really appreciated.