She's a beautiful cat.
I do love tabbies!
Did the vet say it's probably overgrooming?
Is she feeling threatened by the other cats? She was there first and it must be extremely stressful for a cat to suddenly share her space with two more cats, especially if they're indoor pusskins, as they can't "escape" when they want to.
My mum's cat, Solomon's behaviour changed dramatically when he let the two black cats move in (and that was 100% his decision as they were strays). He became grumpy and aggressive, but that's settled now they've all accepted each other. Solomon is top cat, and has never had Red or Yellow vying for his spot in the household. I can imagine there would be a huge problem if one did decide they wanted to be top cat.
Poppy, our cat who we rehomed recently thanks to the lovely Fluffers' on here, overgrooms herself. We've not yet had her a month, so I'm putting it down to the stress of moving and having a completely different family dynamic than at her old house where she was a farm cat. She still curls up in a ball when she sleep, stretches her entire body, comes looking for strokes and purrs like mad, but does overgroom sometimes. The vet isn't concerned yet, but if she carries it on over a prolonged period then we'll need to have another think.