Woodpro offers you a couple of solutions. You can purchase a cabinet with an “Extended side stile and toe board”.  This 3” extension comes factory added to extend 3” on the left, right, or both ends.   The installer would “scribe in” to fit your space.

The other option is to use Filler Boards. These are available in 3” or 6” widths. These separate pieces would be “cut to size” at the job site by your installer to fill in the space.