canesisters
Garden Master
Hey Mary, if you can determine if it was one bird that started it, then removing THAT bird for a week or so might help. When she comes back she will have to work her way back through the pecking order. Sometimes that is enough to adjust a bully's attitude.
Just another option to worry over...
eta: My SS were at the bottom of the order for over a year. Then they got moved to jointly raise some chicks (they had shared the nest and were doing the 'mommy' thing together). When they came back they started kicking chiken butt from one end of the run to the other!! They are now the top hens - and make sure EVERYONE knows it too.
Just another option to worry over...

eta: My SS were at the bottom of the order for over a year. Then they got moved to jointly raise some chicks (they had shared the nest and were doing the 'mommy' thing together). When they came back they started kicking chiken butt from one end of the run to the other!! They are now the top hens - and make sure EVERYONE knows it too.

I'm so sorry it's not working out. Have you considered replacing the 2 'bullies' with a couple others? Maybe some that are a little bit younger than the sussex? Are there any swaps near you? There is almost always one here on Saturdays - if you're willing to drive a little.
Maybe take the two bullies out and put her in the coop to see what happens. Are the two that picking on her new or old hens?