By three copies, we mean that in addition to your primary data, you should also have at least two more backups. Why is one backup not enough? Imagine that you keep your original data on device #1 and its backup is on device #2. There is a risk losing precious documents or photos that are irreplaceable. Once gone, there is no hope of getting these files back. There is a better chance of recovering information if it is placed across a variety of storage solutions.