We are just back tonight from a week in the Pyrenees, and I can now say two things for sure:
1)A third brake is an absolutely necessary item.
2) For a heavy team with heavily loaded panniers, two rim brakes plus one disc may even not be enough.
Let me explain. On all long descents, we use the three brakes in turns to let them cool down in turns. On one long descent, where we lost 300m of altitude in 5 km, S told me that the disc smelled like it was toasted, but everything went fine anyway. Then, the next day, on a very steep and twisty secondary road, I had to keep braking all the time, until we blew out the front tube. Fortunately, only the tube was damaged, the tyre was fine, so we changed the tube and continued the ride.
What we didn't realize at the moment was that the overheating probably weakened the rim tape enough, so the next day, another puncture due to the rim tape collapsing through a spoke hole. I saved the day by putting a piece of old tyre between the tube and the punctured rim tape, and we kept riding while praying for no other spoke hole decides to swallow a tube before we reach the next bicycle shop. Then we bought new rim tapes, and nothing bad happened until the end of the tour!
A