My big oscar fish is beating up on my little oscar and new texan. What should I do?

I bought three oscars at the same time. One of them grew at a very rapid rate. He killed one of the oscars. Now it is twice the size of the other. I tried to put a Texan in the tank to even it that is about the same size as the large oscar to try to put him in his place, but the large oscar just beats up on him too. Any suggestions on how to resolve this.