"three by three by three, what thou dost comes back to thee".
In the loosest sense, karma. A wiccan belief that what you do will be returned threefold. If you do good for others, you will recieve good back--not nessecarily in the way you expect, but in some form. Conversely if you do harm, especially intentionally, you *will* suffer for it, far more than your 'victim' suffered from your action, and again in usually unexpected ways.