There are many variations of the phrasing of the Threefold Law, but it generally goes something like:

Ever Mind The Rule Of Three
Three Times Your Acts Return To Thee
This Lesson Well, Thou Must Learn
Thou Only Gets What Thee Dost Earn¹

When you take a resource, even with good intentions, there will be repercussions. The petty cash box will soon be empty if one doesn't occasionally put money into it as well as take it out. Actions need to be balanced, or else things can get quite out of control to a degree many times over that of the original issue.

In other words, there's no such thing as a free lunch.

Other phrasings emphasize completely separate issues, which author Phyllis Curott has recently taken to calling the Boomerang Whammy Rule:²

Mind the Threefold Law you should,
Three times bad and three times good.

The above version was taken directly from the Rede of the Wiccae, a piece of work that I do not believe is required reading for Wiccans but is interesting to investigate at the least. Or, for an even more extreme version:

Ensure that your actions are honorable,
for all that you do shall return to you, threefold, good or bane.

People attempt to pass this phrasing off as a moral code, which it is not. The Threefold Law is a statement of belief in the ways of the universe. It does not teach us what is "bad" or "good", only that we shall receive three times whatever we give. The only reason it offers for being good is to receive reward and to escape punishment. That is not morality.

The world does not work as simply as these phrases make it sound. If it did we'd all be donating to charity like mad and reaping the rewards by the handful. The idea of things returning threefold is unnatural. According to the Law of Ecology (from biology class - as Wiccans we should be taking lessons from nature):

1. Everything is connected to everything else
2. Everything must go somewhere
3. Nature knows best
4. There is no such thing as a free lunch

But it is true that harm tends to beget harm, and it is true that one good turns deserves another: people remember a person's charity and are more likely to aid them in return. Hence, why I prefer to use the term "Law of Return" over "Threefold Law".

Let's also remember one of Newton's laws as another lesson from nature: for every action there is an equal and opposite reaction. "Opposite" does not mean that you receive bad for every good. It means what gets put out comes back. For instance, if you push upon a wall, the wall is actually pushing back with an equal amount of force - if it did not, it would fall over. That's straight from physics class.

However, what counts as "equal" is not always obvious. Two people trying for identical outcomes might have to exert tremendously different amount of effort to achieve their goals, depending on other factors involved. For instance, a ritual asking for a healthy mother gives birth to a healthy child is fairly straightforward, while one asking for a cocaine-addicted mother to give birth to a healthy child is going to take considerably greater effort, even though the desired result (a healthy child) is the same. (See more in Magic) Putting out a little will gain you a little - which might be all that it needed in the first case, but is far less likely to be sufficient in the second case. You get what you give.

"The Threefold Rule follows the old laws of karma"
Ignoring the "threefold" bit, this statement is actually true. The problem is that most Westerners do not understand what karma actually is. The term karma is Sanskrit and the concept it represents has remained central to Hindus and Buddhists for thousands of years. Moreover, the common understanding of the Threefold Law - one of punishment and reward - likewise has nothing in common with Eastern karmic beliefs.

The Sanskrit word karma really refers to consequences of actions, and so in its proper context, it actually fits fairly well with the Law of Return. The confusion comes from the strong connections between concepts of karma and reincarnation within the Eastern caste system. Depending on one's karma, a person is reborn higher or lower within the hierarchy of castes. However, this is not a system of rewards and punishments. It is a system of lesson learning, preparation, and purity. Those who do not learn the lessons of this life return at the same level. Those who defile themselves will not rise and may return even lower, although there are a number of ways to defile oneself, such as eating impure foods and other actions that we would certainly not describe as evil. Such people's fate is not a punishment, however. Caste is what you are in essence. To be born into a lower or higher caste simply reflects what one has already done to oneself. All of this, however, is merely one expression of karma. The consequences of your actions are reflected in this life as much as in the next, and so it also is in the Law of Return.