I seriously think crowding is your issue, adding birds is only going to make that worse.Both of the other birds seem to be chasing the EE. I don't think I've is more of a bully than the other. I've had them since they were chicks in April. Yes they are my first chickens.
Can you take a tape measure and get exact measurements?
Also what is your climate,
putting your location in your profile is a big help,
and what and how exactly are you feeding?