The words diptych and triptych come from the Greek language, and were often used as terms to describe a work of art that was divided in two or three parts and hinged together. Nowadays, diptychs and triptychs are used commonly in the photography world to stitch photographs beside each other.
Step 1: Open your first image
Go under "Image" and click "Image Size..." to see the width x height dimensions of your photo.
Step 2: Create a new file and set your width
You'll need to double (or triple) the width of your image when you open a new file, making a blank canvas.
Step 3: Drag & Drop your first image
Simply drag and drop your first photo onto the blank canvas, and use your mouse to drag it into place. You can use your arrow keys for very minor placement moves of the photo.
Step 4: Place the second photo and save!
Place your second (or second and third) image onto the canvas, shift it into place and save! Your diptych/triptych is done!