When instructions say plant three feet apart, does that mean measuring three feet from the center of each hole?
Actually, this is simply an average spacing between plants (or planting holes) and does not have to be exact. As long as you stay within this spacing minimum, the plants will be fine. I usually measure from plant to plant, though you can do it however you like. If you're more comfortable measuring from mid-hole to mid-hole, this is fine too.