Rule of Three

Sometimes known as "The law of threefold return".

Any good or bad act a person does will come back on them but three times greater

Sometimes this is applied to magic acts, often by Wiccans. Some people consider it a universal law, similar, but different to Karma.

Commentary
This is traditional, but mathematically makes no sense taken at face value. The more accepted theory is that deeds come back to the user, for good or ill, depending on motive, knowledge, and the amount of energy put forth, and the threefold law is just a poetic way of stating it. I've heard one HPS say that the threefold rule is actually how a deed affects you, your community, and the world at large.