Basic 3D Cube

cube(size = [x,y,z]);
defaults to one corner being at 0,0,0 of the z-axis (flat on top on the x-axis)
cube(size = [20]);
cube(size = [10, 15, 20]);
cube(size = [20], center=true);

Quick Note:

when centered, cubes are centered on the x, y, and z-axes