I purchased a 3 pack system but only needed 2 units since they seem to be covering my total square footage very well.

All the 3 units factory linked to each other?  I was thinking of purchasing a stand alone x20 (or equivilent) and set up a new 2 unit network for my mother at her house.  I know I could keep my 3rd as a potential backup if one of mine goes down, but I'd rather not have it just collect dust in my closet.

What are my best options?