Have your ruler and guides turned on so you can accurately slice your image into three.

1: Open the image.

2: Get your rectangular marquee tool and make a selection about a third of the way along the image. Copy this selection to a new layer by hitting control C then control V.
(If your image is 30cm wide cut it into pieces 10 cm wide.) Do this twice more starting where the previous selection ended. (The first selection will be 0cm to 10cm, the next will be from 10cm to 20cm, and the third will be from 20cm to 30cm)

3: Now you should have four layers, a background and the three slices, turn off the background.

4: Add a new layer. This new layer will be your new background so move it between the background layer and your three slices.

5: Increase the canvas size, I usually make the long end about 10 cm longer and the high end about 5 cm longer.

6: You can fill this layer with a solid colour, or you can have a bit of fun with it like I did.

7: Select your move tool.

8: Highlight the layer with the slice on the left and hit your left arrow key to move the piece left. I usually hit it about 20 times. Do the same with the slice on the right but hit the right arrow, move it the same distance as the left piece.
You should now see that your image is in three panels.

9: Select your magic wand tool and with the right side layer still selected click on the image, then hit Select ->Inverse and this will highlight, with marching ants, the piece. Press Control C and then Control V to copy and paste the selection to a new layer. On this new layer use the magic wand again to highlight the piece, Select ->Inverse and then hit Edit -> Fill and choose black as your fill colour.

10: Hit control D to deselect and move this layer under the first right hand piece. Get your move tool again and hit the up arrow about 5 or 6 times then the right arrow 5 or six times. This should give you a black box behind your right side piece.

11: With the black layer still highlighted go to Filter -> Blur -> Gausian Blur. Set the radius to about 10 and hit OK. Reduce the opacity of the layer so the shadow isn’t to dark, around 50%.

12: Now repeat the process with the other 2 slices.

13: As a final touch you can play with the blending options on the three image layers. Just right click on one of the layers and select Blending Options and play until you get something you like. With this one I just added a bevel and emboss. Once you are happy with the look Click OK. Right click again on the layer and copy the layer style and paste it onto the other layers so they all look the same.

If you have any questions let me know.