It doesn't mean that if you steal a dollar, someone will steal three dollars from you. It's less literal than that. It's an observable tendency, but not usually demonstrated so literally.
To use a physical example of energy: When you expend energy in exercise, this energy is returned in many ways... immediately as sweat, later as increased cardiovascular strength, and finally as overall improved health. (This is presuming you are using this energy wisely, and not harming yourself with over-exercise.)
All energy follows these general guidelines. Some effects of working with energy are immediately apparent (like sweat), some may never be overtly noticed (like cardiovascular fitness), and some may be noticed eventually (like better health).
In a magical sense, a prosperity spell might bring in funds to handle an immediate crisis, but might also help to prevent further financial loss (which you wouldn’t notice) and improve your overall financial future (which you would hopefully notice over time.).
If you are using magickal energy (and we always are, even if we don't notice it), it has ripples that wash against a far shore and then ripple back to you. Sometimes you never even notice the ripples that return to you, and sometimes you get smashed by the wave.
The Wiccan Rede states “Lest in thy self defense it be, ever mind the Rule of Three”. This clear reminder of the consequences of our actions helps to insure high ethical standards in Wicca.
~Flame RavenHawk