First, to make 3D shapes, open a new document, any size, and choose a background. Now, create a new layer. Then, go to filter>>render>>3D transform.
Now, from the buttons on the left side, select either the cylinder, cube, or sphere tool. I have selected the cube tool. Draw several shapes in the middle of your field of view.
Select the same button as in the picture below. Press down on the field of view while rotating your cubes, until they are no longer transparent (until they look similar to below).
Now click OK and you're done! Now you can colourize your cubes, add shadows, text, etc! Here is my finished product:
