Add a shadow along the bottom edges of the top diamond.
Command + click the main object layer and make a new layer. With the selection still active, click the Mask button and drag the layer underneath the other layers for your document. Select the Move tool (V) and without clicking away from the layer press the down button two or three times. Fill the layer with black and set the opacity to 25%. You will now have a shadow for the cubes.
Use a selection of the entire shape to make a shadow area.
If you want to alter the shade like I have, click into the mask area and fill the areas you want to hide with black. Iâ€™ve reduced the left