I would suggest two surrounds. Spaced evenly away from where that single surround was to be. I dont think a single surround would be very useful.
Since you're using your surrounds on the back wall I think dipoles may be useful. They have a null in front of them and most of their sound will be directed to the more perceptually useful side walls.
TV should be as close to eye height as possible so as to avoid neck strain. Speakers should be as close to the same height as the tv as possible.