Cat talk? How to help with fighting between new cats and old cat?

The sub says post pictures, but I'm hoping discussion is Ok too :)

Situation: Have had a cat (Barney) for about 10 years. We brought a second cat into the house that we rescued off the street (Lou) back in 2019, but Lou was not well, and passed away in 2020. Barney mostly ignored Lou, but would occasionally get kind of close to him. If Lou approached Barney, Barney would just walk away and watch him from a distance.

Had to rescue a pair of young cats (about a year old when we got them fixed in January, the vet said), Nox and Sox. They've been in the house since late March or early April.

Barney constantly hisses at them if he sees them, runs far, far away. This attracts the interest of the youngsters, who come running to him to check him out, and this always starts a fight. When the fight begins, Barney runs away as fast as possible, screaming and ejecting as much fur as possible (i didn't know cats could do that, but apparently they can... )

How do I help them to co-exist in the same house? We kept them apart for almost 3 months, and tried to introduce Barney slowly to them once they were comfortable with the environment, but he wants absolutely nothing to do with them so long as they exist.

I don't know what to do :(