Another thought would be to use the Digital audio out as well to access the higher bit rate versions of DD and DTS(which are said to be nearly as good as the HD audio) on the BR discs and not even go into the analog inputs, but to answer your question, yes the receiver will down sample to the 3 channels you have.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70