I had a Powder Blue discus and he was a major bully to this same discus: I have a feeling he is a weak link in the bunch; it seems almost all of the discus pick on him. I hope this works or I may have to get rid of him instead. : (
probably if you got rid of the "weakest link", the other discus would just focus on the next weakest.
You need to add more discus ASAP they will kill each other fairly quickly 3 is the worst possible number to be keeping.
I recommend 1 or 5 or more discus nothing in between