The rule of three is simple but very powerful: Whatever you do comes back to you threefold.
This is not only in magick, but in every aspect of life. Witches are warned in the Wiccan Rede to "ever mind the rule of three." So remember to think before you act, or you could suffer serious consequences.
Ask anyone who did a less than nice spell on someone what repercussions they had. They are never pretty.
Most Wiccans also believe in Karma, a phenomenon that guides the soul toward evolving acts. While it isn't quite a cosmic system of checks and balances, good brings good and bad propagates bad.
Wiccans don't believe in predestination. We make the choices that pattern our lives. We are responsible for our own actions -- we have no one to blame for them except ourselves.
In some religions, there is a belief that you will get your rewards and punishments after you die, in the afterlife. Because of this, many people think they can get away with things, not having to face up to their responsibilities until after death. Not so in Witchcraft. We believe that we are responsible for our actions right here and now. (Raymond Buckland).
Below are some commonly used seals for spells that are evermindful of the rule of three:
By the power of three times three, this spell is bound so mote it be.
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By the karmic power of the number three
This spell tied or knotted be
So that its contents stay together
And can't harm human, beast or weather
(Dorothy Morrison)
*From Bewitching Ways*