I like to arrange in groups of three, which is not necessarily surprising since it’s a fairly common design theme.
Interior designers have long touted the “rule of three” as a tried 
and true method to create harmony, depth and interest. I’m constantly moving pots around and frequently find them in groups of three.
But it is the ubiquity of threes in the garden that are lately catching my eye. Here are a few recent pictures where I had nothing to do with how three plants (or three blooms, leaves or stems) decided to arrange themselves.
Mahonia x media ‘Charity’ (Oregon Grape hybrid)
Iris tenax
Trillium ovatum
tiarella trifoliata leaves
Hellebore leaves
Pacific Northwest Specialty Plants 