Problem with bullying

I'm new at raising chickens.  I have 12 of various breeds,but I'm having a huge problem with 3.  At about 10-12 weeks old one of my chicks became the victim of severe bullying from the other 9 birds.  I believe this was a result of overcrowding.  I separated her, let her heal & have reintroduced her to the rest of flock in a new & improved coop & run.  It's been about 6 weeks or so & she still has not become part of the flock.  About 4 weeks ago, I acquired two younger Marans & added them to the flock.  These 3 birds are constantly bullied,  I understand there is a pecking order, but I'm concerned that these 3 aren't getting enough to eat.  I've placed additional waterers & feeders in the coop, but they get chased away from these as well.  I welcome any thoughts or advice!

If you can identify a ring leader you should remove her for a couple of weeks before putting her back in - if you can house her within sight of the others that helps when you re introduce her. Otherwise you can give them more time and /or space or house the bullied ones separately. Good luck