Lets get some basics straight here stumpy, you're not being very clear with us and as such it's impossible for us to give any decent advice.
Give us a list of what tanks you have, what size they are and what fish is in them please. Also if you can tell us how long they've been running for and what the water parameters for pH, ammonia, nitrite and nitrate are in each tank.
In general terms it is highly unadvisable to keep three Oscars together, when they get older (around 1 yr old) they will become very territorial and agressive against their own kind. Inevitably one of them will be slightly smaller and weaker than the other two, they will systematically bully him until he either dies from his injuries or contracts a stress related illness and died from that. Once he is dead the other two will turn on each other and whichever turns out to be weaker will go the same way. When keeping Oscars you have a choice, you either keep 1 oscar with no other oscars or you keep a group of 6 oscars or more. You can also keep a breeding pair together, but be warned you can't just expect two oscars to form a pair, they are picky about their partners and if they do not take to each other they'll kill each other so you can't just put 2 of them in together and expect a pair to form.
Now to keep a single Oscar you need a tank of 75 gallons as a minimum, to keep a group of 6 Oscars you're looking a a tank in teh region of 500 gallons...... that is a seriously big tank, so unless you have something of this size then you need to return two of the Oscars to the store, don't just swap them for anything that takes your fancy, you need to do some research into suitable tankmates, once you have given us the tank size that the oscar is in we can make some recommendations from there about what you can put in with it, that's assuming that the tank is big enough for Oscars at all.