What does it mean when you are told to plant in threes?

Planting in odd numbered groups is done to provide a more natural, non-linear look to the landscape.  Instead of rows plants are grouped in clusters.  Three is the smallest group size that will give you a non-rectangular shape (triangle).  Even in the most formal of gardens
you will find odd numbers of plants, with a center focal point and balanced plantings on each side.  For most people the results of planting in odd numbers is most pleasing to the eye