HI Julie!!! It's nice to see you posting. I wish it was for better reasons though.
Wow! you have a very serious problem there and I really don't know what the solution is. I can only relate this back to my personal experience with Magoo. Is there anyway you can medicate Mr. Nasty? Could you somehow slip him some canned food laced with some meds to calm him down? In Magoo's case it only took a few months of medication for him to relax. I don't know if that would work for this guy or not. I realize that this could not go on forever but it is possible that it could work as a temporary solution.
I think for Magoo it was an adjustment time but I also think he had been an intact male for so long that aggression was just the norm for him. A few months of meds made him change his ways and he found he could be friends with other kitties and it was ok. I wonder if that could happen for Dilly too?
I'm sorry but it is the only suggestion I have. With Ally there is no way you can bring in the other two, but it is also terrible for them to constantly be seriously hurt by Dilly.
I wish I had more advice to offer.
{{{HUGS}}} to you and I know you will do what is best for everyone involved. Good luck and keep us posted.