Answer: Emerald Isle typically grows 6-8' wide. I'd plant them on 4' centers so they will grow together without crowding. You may have difficulty keeping them at a 3' width as they age - 5-6' would be a better goal. If you prune them too narrow, you'll end up with some bare areas along the trunk. Arborvitae would be a better choice because their mature width is only about 3'. If you have the room, go ahead and plant the Emerald Isle's. If not, you may want to choose another plant.
Best wishes with your landscape!
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