The Rule of Three is that That which you put into the world returns to you three times.

The statement comes from The Wiccan Rede (Counsel of the Wise Ones), the history of which I discuss elsewhere. The couplet is:

As such, it should be read and considered as poetry rather than mathematics. Besides -- how would one go about measuring such things?

As I have mentioned, Wicca is an occult relgion, and one of the ways things are kept hidden (occulted) is that lots of meaning is crammed into a few words with the expectation that the Seeker will devote time and effort to unpacking it. (And if they won't, a handy vehicle is provided for them to announce the fact to the world when they loudly proclaim their analyses based on the most esoteric and asinine interpretations possible!)

Numbers have significance in Magick, and the details of that are a topic unto themselves. Libraries can be filled with the books written on that topic. For our present purposes, accept that the number three means "lots." The number three figures in here for a number of reasons:

• That which you put into the world returns to you recursively. Feedback, man. Largely because...
• That which you put into the world returns to you by several paths. To do Magick, it is necessary to touch three things:
  • That Which Acts Upon
  • That Which Is Acted Upon
  • That Which Is The Medium of Action
• That which you put into the world was within you to begin with. It's still there, much in the way that a candle that lights another candle is still lit.

Those who wonder about things like conservation of energy should recognize that there is no energy being created in the Triple Return, it is the same energy encountered three times from three perspectives.

Also note that this is not in any way shape or form some sort of morality police going about making sure everyone gets theirs. This is simply how the universe is structured. Getting wet is not a moral pronouncement.