I've looked and looked into the maintenance manual (not a pilot - sorry); I reallly can't figure out why this would happen. All three systems (capt/FO/stby) are completely independent. Even the single tuning panel receives 3 seperate power sources internally. I also don't see how the 757 could have been certified autoland cat3B if you loose all ILS's when 1 of the busses fails.
Where did you see such a situation, is it described as such in the FCOM?