Bob, this is one area of carbon printing I have no personal experience with. The late Gordon Chappel showed me his set-up...he used anywhere from a single layer to 4 or 5 layers, and I am trying to remember what he used as a final support material (non-shrinking). But I am not being successful. I wonder is just pre-shrinking the paper (soaking and drying a couple times) and then taping it down on all sides to the registration board might be enough...but this is just me thinking aloud. The synthetic watercolor paper, Yupo, may not shrink.
This would be a good question for the folks over on the carbon printing forum I linked to earlier. Sandy King started off do 3 color carbons, so he would be able to give some solid advise over there.