You'll have to render the renderlayers B4 you can work with the file. Just connect the image output from the second z-combine node to the composite and viewer nodes, then render. No need to connect the z output as the value gets passed through anyway. You'll find the renderlayers to the right of the composite node. After rendering reimport the render into each of the image layers. The image layers were configured from top to bottom: Rear, Center, Front, but you can order them any way you choose, but that will give you a different effectthan what you see in the screen shot. These files are HUUUGE! This particular file was rendered at 960 X 540 and cost me 16.8 MB. Each of the 3 layers was rendered with 8 passes. No effects were applied during render even though passes were enabled. All post was done upon reimport of the multilayer. Be sure to deselect single on the renderlayers tab B4 rendering, else you'll only render the layer labeled "Front".
If nothing else this is just fun to look at. RamboBaby hopes that it will help you learn how the system works.