Hi and thank you for the compliments about our work! Generally speaking we typically follow the rule of odd numbers for all our plant groupings. You are correct that the grass can work well in groups of three, especially larger grasses, but we will also use them in groups of 5 and 7. Small grasses may even be massed in larger numbers than that acting then almost as a groundcover. For perennials we usually don't do groups of less than 5, unless it is a larger perennial like a daylily or a hosta. Hope that helps!

James