The Law of Three or The Threefold Law is a very good law for witches to follow. Basically it is a law of cause and effect and it says that if you work magic it will come back to you times three. So if you use magic with good intentions (EX: Healing) the positive energy will come back to you three times stronger. But if you use magic with bad intentions (EX: Revenge) the negative energy will come back to you three times stronger.
This is not to say that witches do not wish revenge against something that has happened, it just means that they know that their actions against it will not help the situation at all. Just like Karma, they know that what is meant to happen, will happen. And The Goddess and God will take care of it, so we do not need to get involved and make the situation worse then it needs to be.
Wiccan beliefs assure that what is merited by a persons actions will come to pass. You need only to ask the God and Goddess for "justice to be done". This is in line with natural laws and the promises of the Ancient Ones. Once you have asked in the correct manner you can rest assured that the situation will be taken care of. The issue can then be released and you can go about your normal business.