Q: What do you call a 3-hedron? I was just reading about polyhedrons and wondered, even though it's an impossible shape, what a 3-hedron is called?  I'm really asking what the Greek for '3'.  Does it just converge to a triangle?

A: Tri- is the greek prefix. 
We do use the word trihedron, but to refer to something slightly different: the Frenet-Serret trihedron.
