I have 3 oscars in a 120 gallon tank. my tiger and smaller red oscar harass the larger red and it almost never comes out of the corner except to eat. are they a mating pair or what might be going on?

How old are the Oscars? There is certainly a possibility that they may pair up and pick on any other Oscar in the tank, this is quite common. However, some Oscars hang around with each other as if they are a pair but never breed, they could be a couple of males, or two females. Time will tell

Agreed....keep an eye on the situation, it could turn nasty,so be ready, is there any aggresstion from the two towards the one?

The oscars are about a year old