After you've saved the main image as a PSD (File > Save As), you need to do the following:

• flip the last image (Image > Image Rotation > Flip Canvas Horizontal)
• select the images (Select > All)
• copy them (Edit > Copy)
• paste (Edit > Pate) into the main image.

After these steps are complete, you need to expand the canvas (Image > Canvas Size) to 1200 pixels.

Finally, move the two pasted images to either edge.