Well there's your problem. At their current size (I'm guessing no larger than 3-4" if they all even fit in a 6 gallon), they need to be in at least a 30 gallon tank (which will last about 3-4 months before a larger tank will be needed). Your final tank would have to be at least 125 gallons for 3 oscars (they grow to about 14" on average, although yours are probably stunted to some degree, depending on how long they have been in your tank). So unless you have the space/money to upgrade, you should probably take them back and get some more appropriate fish for that tank size (I guarantee you they'll all be dead in a a couple months if you keep them in that tank).