The "textbook" location for the rear center speaker is on the center of the rear wall, at the same height as the side surrounds. The second subwoofer doesn't have to be a "rear sub" -- the subwoofer sound is not and should not be localizable. In other words, all the subs do is to excite the entire room with bass frequencies. The optimal placement for the two subs can only be determined by the actual try-and-error. Good places to start with are two corners of the room or along the opposing walls (sides or front/rear) near their centers. As for the center channel, what you should aim at is to have the front L/C/R at about the same distance from the primary listening position. So, rather than on the same plane, the center should be placed so that it makes an arch with the front L/R. That said, most of today's AV receivers have channel delay (speaker distance) adjustments, so this is not absolutely critical.
