Sorry, this may be long but I don't want to miss anything out. We had 4 cats, who all got along reasonably well, O was 'top dog' but became elderly and S started picking on him, we managed this and O died in February.
We now have 3 cats, R went for a dental extraction, when he came home S was very aggressive to him, hissing, growling, proper cat fights. We separated them until R had recovered.
Now, about 3 months after the dental, S is still very aggressive, R is scared when S is around and hides, S hisses and growls often. He also does this at the third cat T but not as much- although T isn't around as much as R.
We don't know what to, it isn't getting better.
All neutered males, S is 8 T is 10 and R is 14 all in good health.