Why does she chase them? And what do I do?

Hi everybody!

I posted a few weeks ago about our new adult cat, Lucy, that we took in about a month ago. We are having a few issues with her getting along with my 2 other adult resident cats. Some things have changed since I last posted...It's gotten slightly better. But there is one main problem that really makes it veeeeery hard for the 3 of them to start to get along...

Lucy has a habit of ALWAYS chasing Ernie or Dolly if they do emerge from under the bed or get down from a high place. This is the main reason why we have to keep Lucy in a seperate room the majority of the day. Otherwise, my 2 resident cats would never be able to get to places they wanna go in the house without getting chased (which results in them detouring and hiding instead.) I wish they would just stand their ground but they don't.

It's NOT playful chasing when Lucy does this. I'm very sure of this. She's either doing it because she's scared and she's chasing them away in anticipation of something, OR maybe she's trying to be dominant? I don't know...

WHY is she doing this?...and HOW do I prevent her from doing it?

If she would just stop doing this, things would probably get better much quicker... :(