I assume you have a wide screen tv, therefore your viewing position is pretty close to the back wall and you only have the front wall, left side, and back wall. With this configuration you are forced to have 5.1 set up.
Having this in mind, you are better off with 2 QS4's mounted on the back wall. I do not see the advantage of placing a third surround when the distance between the 2 surrounds is less than 12 feet and you are already seated pretty close to the back wall.
By the way do not forget a sub. Your set up will sound feeble without one.
