Using the mirrors at the gym in Oasis Springs as an example, starting with the 3 panel wall mirror you have 2 seams in your wall mirror.
Now turn on the moveobjects cheat and get the single panel mirror and place it so it is over one of the seams to hide it.
Now just do the same thing for the other seam and you now have a wall mirror with no seams showing. Which I think looks much nicer.
You can make any size wall mirror you need by using a combination of the 3 panel and single panel mirrors. Like if you just want one that is two panels wide place two of the single panel mirrors side by side and place a third one in the middle to cover the seam.