The rule of 3 in landscaping means planting in odd-numbered groups, usually threes, and repeating a plant, color, or texture three times across a bed. Odd groupings read as natural drifts instead of soldiers in a row, and repetition ties separate areas of the yard together so the design feels intentional rather than collected.
Designers also apply it vertically with three height layers: ground covers in front, mid-height shrubs behind them, and trees or tall grasses at the back. A practical starting point is three of each perennial, spaced at mature width, rather than one each of nine different plants.
If you want to see how grouped planting changes the look of your own beds, test a few variations with AI landscape design before you buy anything.