The Rule of Three, also known as the Three Fold Law, basicaly says that whatever you do, good or bad, will come back on you x3. Basicaly, Karma. Don't take this in a literal way. Just because you punch someone in the eye for no good reason, that doesnt mean that you're gonna get punched in the eye three times, but you might accidently break a leg or something else that could be considered three times worse then getting punched in the eye. This applies to magickal actions as well as well as mundane ones. Let's say you and Tom are both trying out for the lead role in your school musical. You cast a hex on Tom to make him get a nasty cough and end up losing his voice so he can't sing. You might then, for example, catch pnemonia and be uneligable for the part as well. In this situation, you could have worked magick on yourself to help you do your absolute best. Oh, and just so we are clear, you can't cast a spell to make the drama director pick you. That would be messing with another persons free will by making them do something they wouldn't normaly do, and that is also a no-no.