My rats don't get on anymore. The older one has a cage of her own now but may be lonely. Ideas?

I have 3 rats. The eldest was very lonely when her friend died so I got her 2 new friends. They got on quite well for over 6monhs but recently they started fighting a lot. I have separated them to find out who is causing the problem and I think it was the younger 2 being to boisterous with her. So now she is on her own and a bit lonely. Is it worth adopting an older rat to keep her company do you think?