So very sorry to hear this.
You might want to spend time closely observing which of the 2 month old chickens are the instigator. There might be one who is the bully. If you can figure this out, remove her and then see how the smaller two (if the injured one survives) do with the 'non bully' older chicken. If it goes OK, completely remove the bully chicken from view (dog crate in garage, etc.) for a week or so, then try re adding under supervision.
And sometimes they need to live 'in parallel' for a longer period of time. Some people start integration by cutting a couple of doors that only the smaller ones can get through, so they can run back to their 'safe side'.
(Note; If you need to nurse the injured chicken inside, then bring the other one with it for now, as it is likely to recover better with companionship).
An awful thing for you to find. Hope the injured chick makes it.