Draw a square.
Extend this square downwards and to the left to form a cube.
With the front square of this cube extend it downwards and to the right to form a second cube (the front face of this cube should be in line with the fist square you drew)
Now extend the front face of the second cube upwards and to the right to form a third cube (the back face of this cube should be inline with the front face of the cube you first drew)
Finally join the corners of the back face of the cube you just drew with the corresponding corners of the square you originally drew.