Need help with cat aggression issue please?

I have 3 cats, all neutered males - have had them for over 5 years. The last couple of days, the 2nd oldest cat has been attacking the oldest. They've been vicious, blood-drawing fights that are always started by the 2nd oldest cat. I don't know why he's doing this or how I can stop it. Would really appreciate any advice. I cannot even fathom getting rid of this cat, I love him too much, but I can't let him keep attacking my oldest. This morning it was so bad I had to take him to the vet and he received 4 stitches. I've locked the 2nd oldest cat in the spare bedroom for now, but I really need some help here. PLEASE no rude comments. Please please please. I'm very upset and desperate to fix this. Thank you.