Explanation:
in 3-dimensional math we have 3 dimensions, and not just 2.
we have
forwards - backwards
left- right
up- down
so, we have there x, y and z.
and the equations typically are
z = ax + by
or even more general
cz = ax + by
or more complex functions of x and y.
z = f(x, y).
I am not sure what else you could mean with that question.