A cube consists of 6 equal squares joined at right angles in a three dimensional space where X is the horizontal axis, Y is vertical and Z represents depth.
So in the diagram below where the side of each square is a length of 2, one would define vertices A, B and C in terms of (X, Y, Z) as follows:
A. (-1, +1, -1):
B. (-1, -1, -1);
C. (+1, -1, -1);
Note that every Z value on the front facing square will be -1; the Z values on the rear square will be +1.