Generally, the advice here would be to swap your LNBs for Twin-outputs, replicate your DiSEqC setup and add the extra cable running to your third box keeping the original cabling setup as it is, that's all I could recommend...
Oooor, there's the good old Manual A-B switch, put this before the Auto-switch on the "A" port, and hook up the extra box to the "B" port, that could work...