This triple gear is a real thing, and thanks to some intricate math and the advent of 3-D printing, it exists. Before this, at least as far as I can tell, a triple-meshed gear required one of the gears to turn in the opposite direction as the other two. That is no longer the case.
I can’t for the life of me imagine what this would be used in, but hey … at least we have it now. Get to designing!
(via henryseg on Shapeways)