He attacked because he is in someone elses territory. Theres a lot of stress and tension, which caused him to lash out. Its a fear response.
If you want a trio, youre going to have to split Eta and Dicky. If they cannot handle their cage next to his without them circling, youll need to split them. Each bunny needs their own cage, and you need to have all three cages set up so they can see and smell one another, but cannot reach one another. You can set their cages up in a sort of triangle with the corners being 3 to 5 inches apart. Youll keep them in this set up for a month.
After a month, youll start swapping their cages. Like this:
Day 1: Bunny1 in cage1, Bunny2 in cage2, Bunny3 in cage3.
Day 2: Bunny1 in cage2, Bunny2 in cage3, Bunny3 in cage1.
Day 3: Bunny1 in cage3, Bunny2 n cage1, Bunny3 in cage 2.
Day 4: Bunny1 in cage1, Bunny2 in cage2, Bunny3 in cage3.
Day 5: Bunny1 in cage2, Bunny2 in cage3, Bunny3 in cage1.
Ect, ect, ect.
Youll do this swapping until they all calm down about it and stay calm for a solid week of switching.
After this swapping, youll start bonding in a small neutral space. Many people use their bathrooms or their bath tubs. Youll start with a lot of quick sessions per day. Put them all in this small neutral space for 2 minutes, then bring them back to their cages. Youll do this about once an hour. The second day, increase time up to 5 minute sessions every hour. Next day, 10 minutes every hour. Then 20. 30. 45. An hour. Ect Once they can go 8+ hours together without aggression in this small neutral space, you can start bonding sessions in a semi neutral area, like the room they are sharing. Repeat with many short sessions slowly increasing until they can spend 8+ hours together without aggression.
You can bond all three at once, or start 1 on 1 sessions between two of them, alternating between sessions. Only increase steps once all three together can spend 8+ hours with no aggression.
If you dont want to split Dicky and Eta, Im not completely sure how to go about this. If they cannot handle Chester being nearby without Dicky getting aggressive at Eta, it means that Chester being there is disrupting their bond. I think if you want a trio, youre going to have to split them. If you dont want to split them, I dont think there is a way to make a trio work.
Outcomes of bonding a single into a bonded pair:
Bonded pair reject the single, leaving you with a single and a bonded pair who cannot bond.
Bonded pair reject the single, and also reject each other, leaving you with three single rabbits who cannot bond.
Bonded pair reject eachother, but one of the bunnies bonds with the single, leaving you with a new pair and a new single.
Bonded pair welcome the single into their bond, leaving you with a bonded trio.