Cat Aggressiveness? How to help-treat-cure- or medicate Aggressive cat behavior?

We have a 17 year old male cat that growls, and attempts to viciously attack the two to new cats that we’ve adopted. What are some possible solutions to cure this kind of aggressivness? They literally cannot be in the same room with eachother without a stand off!
-All three of the cats are adults
-All three cats are males
-All three are fixed

Please help, thanks.
I wanted to add….
My 17 yr old cat has always had this behavior.. I’m just looking for solutions to help suppress some of his viciousness. He has always been like this towards cats; his aggressiveness has always been Extreme! This isn’t a recent personality alteration. I took all of these factors into account before adopting these new cats. I didn’t get these new male cats for the simple fact of having new cats around. I actually rescued them from an unsuitable living condition.