There are switch boxes that can be purchased (you could use three two-channel stereo boxes if that is all you could find), and the higher end receivers for the past couple of years have more than one 5.1 analog input (one of which is at least a 7.1 input).
Also, the high end SACD and DVD-A players can use a special digital connection, and the high end receivers (of the same brand) can accept this, so only one 5.1 analog connection is needed in such cases.
Really, I think this is not likely to be a problem for most people, as SACD and DVD-A have not caught on much.