When females do not get along it can be a very difficult problem to resolve. What you have done is exactly right, give the victim a small place of her own. Food., water, bed, litterbox and your attention when possible. She will be much happier there even though it seems to you like she is alone and isolated.
If you decide to keep the new cats you could put them away at times (not in the victim's room) so the victim can be out with the boys and you again.
Finding them another home is certainly an option as well, but that has to be your decision.
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