The designers at Everything Grows often put indoor plants in groups of three.  From a design standpoint, using groups of threes helps build interest and drama, as well as harmony in your space.  Objects that appear in threes are more appealing, memorable, and effective than other numbers of objects. 
Here are some examples of groups of three that we have recently installed:

The three photos below are groups of three just off the elevator landing, with each floor being a different color (orange, blue, or gray) to help visitors identify which floor they are on.  It is very effective and dramatic.