I also call it blending 3 photos.
Okay, first choose 3 similar photos. They don't have to be taken at the exact same place and time, but it helps for a more realistic merge, otherwise, it's just a blend.
Here are my photos.
First, create a new artboard. I made mine 15 inches by 5 inches tall and 300 dpi. Then drop all of your pictures into the new artboard. A good practice would be to save it now! (My computer crashed 3 times while creating this tutorial, and I should learn by now to save stuff right away.) It's good to save after every change you make that you are satisfied with.
Next, do any rotating or enlarging now. Make sure the layers overlap a bit.
The leftmost image should be the top layer.
You are going to create a layer mask by clicking the black square with the white circle in it, in your layer pane.
Then, using your gradient tool (make sure you are working on the layer mask) Make black your foreground color and white your background color and drag a gradient from the middle of the center picture toward the left picture, about the length of the overlap. (this is trial and error, keep redoing the gradient until you get a result you like)
Take the paint brush tool (make sure you are still on the layer mask) and at a lower opacity (I used 25%) pain over where you can see the *seams" of the photo to create a smooth blend.
To merge the other side, make sure the rightmost image is the middle layer. Create a layer mask, and this time use white as your foreground and black as your background and draw a gradient from the center to the right about the length of the overlap.
This time, use the eraser to erase the photo seams.
Flatten the image, adjust the color and...
Voila! Photomerge.
Now you can add borders, text, etc.
