Except the 3rd one has been pressed into service when one of the other pair have had to undergo maintenance in Zurich (e.g. around 3 weeks ago) and when either aircraft has gone tech (e.g. December 2001). It was also used to "rescue" the schedule when one of them was running very late inbound to MAN.
Given that Air Atlanta normally have one or two 747s sitting at MAN, I guess that they may be used should remaining one need to have longish downtime.
For short-term problems, would they be allowed to put passengers on one of their flights down to LHR and depart on a UA flight instead?