Cats still not getting along after 11 months

3 male cats. All neutered. Ages 12, 3 and 3. Two are brothers that I have had since 2 weeks old. The older of the three is my partners cat, she moved in with him at the end of March last year.

We followed all suggested advice when introducing cats. Kept them separated, swapped them out of rooms so they could smell each other and then eventually put up a screen so they could see each other but not have access to one another. After using the screen for a while, all the cats seemed pretty mellow so we tried an introduction.

This didn’t go great since my two young guys just bullied the older one. If they aren’t going after him they tend to just stalk him.

We have tried playing with the cats as a distraction but they are so focused on the older cat nothing distracts them.

Do we need to go back to closed doors and smells for now?

I intend on checking out some videos on YouTube, maybe Jackson Galaxy’s since I know he has good info. Any other help would be appreciated.