What's the 3-3-3 Rule you ask young grasshopper?
It's a design principal for gardening: one third perennials, one third deciduous shrubs and one third evergreen shrubs.
The key to this combo is that 1. throughout the year, you have the presence of an 'ever-green' - a plant that keeps its 'leaves' or rather needles, 2. you have the deciduous shrub that sheds its leaves but provides a strong focal point for winter interest with lovely branches and 3. of course, your reliable perennials that fill in the gaps, create pockets of colours - whether from flowers or foliage - and last for years with generally little input from you, if planted appropriately.
Another landscaping rule of threes is to do with the number of a particular plant. Like everything from math to home decor, our eyes like to see placements in odd numbers, and three most often fits the bill. Instead of having multitudes of different plants dotting your landscape which distract the eye, causing an chaotic look, a key design trick is to buy multiple numbers of the same species and planting them at either uniform distances apart, or in large, pleasing drifts.
A final trick to landscaping is layering. Layering involves designing your garden beds so that you start with trees, tall shrubs and large perennials as the backdrop, smaller versions of these planted in the middle, with the border made up of smaller garden plants, and some annuals.