Well Ian had said 4 subs is nearly ideal so 3 would be 3/4 to ideal. I would think the other 2 2/3 of the way down the side walls would work. You could also try one 1/2 way down and the the other all the way to the back just see which works better in your situation. After all it is about building up the low points so positioning is the key.
I think you are going to need to do some work once you get them. It is all just speculation until they are installed.