Aww how heartbreaking for her and you.
I don’t have experience with trios, but I’ve heard this can happen, where one bunny gets shunned. For now you should keep her separate, as if there is fur on the floor that means they are scuffling and it could get very serious. She could have been shunned because of her health, or because she was separated from the others for too long, unfortunately. If it’s possible to keep her in sight of the other two, it should be comforting to her to not be alone.
I can see two routes: take a break (since they scuffled) and then start over with re-bonding the trio in neutral ground. If they shunned her just because she was away for too long and they forgot about her, then it might be pretty easy to rebond them. But, if it was because of her health, might be harder for them to accept her.
Another option (if it doesn’t seem like rebonding the trio will work) would be to try to rebond her with a new bun, and have two pairs. It sounds like you have a lot of space, so you could divide the area into two separate living quarters for your pairs…
Sorry again, hopefully you can get them all bonded again!
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.