The more fundamental question is "why do you have Coord3D class?" Why not just use a tuple?
The general advice most of us give to Python n00bz is "don't invent new classes until you have to."
Does your Coord3D have unique methods? Perhaps you need a new class. Or -- perhaps -- you only need some functions that operate on tuples.
Does your Coord3D have changeable state? Hardly likely. An immutable tuple starts to look like a better representation than a new class.
