I encountered a bike with a double front mech and a triple crankset a few months back at the co-op I volunteer at. It was a late '80s MTB, and I never got a chance to see if it worked, because the little nub for the spring on the derailleur body had sheared off, so the derailleur didn't move back when the cable went slack. I can only assume it worked at some point, as all three chainrings were worn. The granny ring had about 30 teeth, as I recall. It also had friction shifters, which was probably a good thing, as I suspect it would have needed a lot of trimming to keep it running smoothly.
tl,dr A double front mech CAN be used with a triple crankset, but it'll probably shift like crap and may not work with indexed shifting.
Last edited by Airburst; 02-13-12 at 11:21 AM.
Reason: clarity