On doo triples, it fires all 3 plugs 3 times for each engine rotation. So one cylinder is actually were it should be when it fires and causes combustion, while the other 2 are just wasted sparks.
I found this out when I was using my little digital tach to calibrate the tachs on the sled. It worked fine on the twins and when I put it on the triples, the number was about 50% higher than what I expected.