If you struggled on a triplet keeping up with two-person tandems downhill, then I would agree, it has to be the drag from the tires. On my triplet (running 28 x 700), I can't remember ever being passed by a regular tandem downhill.
As for the on-going 26" vs 700c debate. I've read all the arguments on both sides and cannot figure out a legitimate reason why there would be a speed differential. As far as I can tell, all evidence either way is anecdotal (often justifed with some very persuasive scientific reasoning). I wish some company in the industry would do a true scientific study of wheels, tires, speed, braking, etc. Perhaps the tire manufacturers all have too much invested in the status quo.