What can I do if I need another gang in a junction box but there isn't space in the wall?
If there are studs at either end of a two gang switch box, that means they are approximately 4 inches apart.  A three gang requires 6 inches.

A switch box is usually about 3.5 inches high; with thickness that means turned sideways, it will fit in a dual ganged space.  So you have room for 20 switches if you want:



(from here)