How do I make a 3D cube?

Well, it can be done.

Inventory

1. rectangles, 2
2. parallelograms, 4

Exploded diagram: Completed:

Individually, you will want each shape object selected, and from the Arrange tab, set Text Wrap to None. This will facilitate object alignment later.

Insert one rectangle, and duplicate it by option+drag to an offset as shown above. Duplicate that last rectangle again with an option+drag as it will become your playdough for the first parallelogram shape. Select the third rectangle, and then from the Format menu, choose Shapes and Lines : Make Editable. The four corners of the rectangle will now have red grips, which you can pull, and push to arrive at the first parallelogram shape. Click outside of the shape to dismiss the red edit grips. When you have an approximate parallelogram shape, option+drag once to make its opposite cube wall, and then option+drag to make another that you will flip (via the Arrange tab), to make the top, and then option+drag to make the bottom shape.

At this point, you have the exploded diagram above. Don't worry when you drag the pieces together and they don't exactly fit. Once again, you use the Format menu : Shapes and Lines : Make Editable to fine tune your parallelogram fitment in their cube side locations. When I was done, I selected the entire cube contents and applied 0.5 pt grey border lines, and every cube face is 38% opacity. Color fill for each cube face is optional.