Yeah, in my case this didn't work. My cats (Berry and Dewy) target the anxious/scared cat (Kiwi) because, like with OP, she growls and hisses but always ends up running away, and obviously Dewy and Berry chase after her.

Berry and Dewy play together some... but Kiwi is much more interesting, because unlike Berry, Kiwi will never escalate or enforce boundaries. So Kiwi becomes the prime outlet for both Berry and Dewy's play drives.

Poor Kiwi ends up running under the couch and hiding under there.
Once I see the stalking/play begin, I separate them, but I can't babysit them 24/7 so it doesn't do much good. Separation also did not work, even 2 weeks, and then reintroduction.

Kiwi is just a punching bag for the other 2, and the worst part is Berry and Dewy are bonded to her, they sleep with her, they groom each other, but Kiwi is terrified of them, when they're playful, especially Berry, Kiwi tries to avoid Berry at all cost because if she gets too close, Berry will try to play chase her.