Hello Teresa,
I had the same problem and my Penny had only been absent from her flock one week.
It is more difficult to get a flock to accept only one hen back, I think absence for 3 months will take longer than my ordeal, but it can be accomplished.
The link in my signature below is an article I wrote about a successful reintegration of one hen to her flock - hope it helps.
I had a lot of advice that really did work - can you post pics of your "arrangement" the run, coop, and how you are separating the BR?
The Bully blaster water spray works immediately to bust up aggression, I used a spray bottle with a long shot of water, others use a garden hose, either can work. Some use a can of rocks to shake to break things up - I did try a can of marbles, it only worked once.
The flock's aggression will appear whether you are present or not, so if you are diligent in the process, you will begin to see the aggression lessen and at that point things get easier. If you flock free ranges it would be easier - mine live only in confinement which compounded the difficulty of the process. I added an additional make-shift interesting out of the run area so I could lock out the more aggressive hens at first and allow the lesser aggressors to accept her. It took months of daily close observation, having the recovered hen live in a dog cage in the run so she was safe, had her own water and feed. And a jail for the worst bully to spray her and isolate her so Penny could get some time with the friendlier hens. It's stressful to us and more so to the one who needs to reenter the flock. Do not try to just put her in the coop at night - that was a real disaster. Be patient - it takes time to accomplish.
I think the longer the hen is removed from the flock, the longer time you have to practice the see but don't touch method - Penny was only away 1 week - with yours removed 3 months, it could take longer?
We are here to help, let us know how your daily integration process is going.