Answer: First of all, I'll describe some of the plant's requirements. Arborvitae should be planted in fertile, moist, well-drained soil. They like full sun, though light shade is acceptable. (In very shady spots they'll lose their internal foliage and become airy and open.) Certain cultivars are particularly susceptible to winter damage in your zone, despite winter protection.
Now let's talk a bit about pruning. When the tree is small, pruning is generally corrective, such as pinching buds and redirecting branches to get the plant to grow in a strong, attractive shape. In the prime of life the shrub would probably benefit from some corrective pruning for rejuvination, beauty or usefullness. In its old age, you just want to keep it healthy enough to live long, and well.
At three years, I would assume yours fall kind of in between "young" and "prime"! This means you need to be helping it look good. Evergreens make their growth from buds formed the preceeding year. Proper pruning or shearing temporarily stops the active growth of the large new buds, but stimulates the sprouting and growth of many of the smaller ones that lie dormant all over the branches. Because of this, when you shear off the new growth, the tree's energy which would normally be directed towards those few large, buds, now is redirected into the thousands of little twigs, and you force the tree to grow bushier!
For the best results, begin the first shearing soon after the growth starts in the spring. Don't allow the tree to make a lot of growth before you shear it, or the smaller buds will remain dormant. Also, when you cut during the early part of the growing season the cuts will heal quickly and new buds will form where the cut was make. These new buds will grow in the spring, hiding the shearing wounds. If you shear too late in the season, the newly formed buds will be cut off leaving unsightly cut stubs showing all year.
Finally, consider mulching the plants each spring with a layer of compost.
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