So what you actually need is a speaker swithcing box, I think.
One pair of speaker wires in and 3 out - but then that's going the wrong way!
hmm not sure...
QED make them the other way round:
http://www.audiovisualonline.co.uk/dynamic/eshop_products.set/ref/1035/qed-ss30-3-way-speaker-switching-box/display.html?referer=1
http://www.superfi.co.uk/index.cfm/page/category.cfm/category/31
not sure if that helps, its something of an unusal setup