The Three-Fold Law is simply the principle that whatever you send out returns to you three times.
It's not a law like a traffic law, it's a law like the Law of Gravity. You can't break it, no matter how hard you try, and there is no way around it. It's simply true.
Anything you send out into the Universe is reflected back, multiplied.
In practice, that means that if you are nasty, you will get three times as much nastiness back. If you are kind, you will get three times the kindness back. If you are stingy, other people will be stingy to you. If you are generous, they will be generous. You get the idea.
It's been known for a long time. You can see it at work all around. People tend to have friends who are basically the same kind of people they are. If you stand in a bank line, and watch the people, the friendly ones have friendly tellers, the business like ones have business like tellers, and the ones who want to argue have tellers who don't want to help at all. The tellers are the same; only their attitudes reflect the customers. (Banks are a good place to watch, because the same people go in all the time, and the tellers get to know them; but it happens everywhere.)
If you don't believe me, try it in your own life. Smile, even if you don't feel like it, and greet everyone as if they were a real person. Be friendly and polite. Betcha that before the day is over, you will really feel like smiling, and that people will be much more likely to want to help you.
It works on a larger scale, too, although it's not as easy to see.
I've had someone tell me that it can't be true, because the sharp, nasty business man is accruing mountains of wealth, while the really nice old guy on the corner is barely scraping by on his Social Security check. But I think they are looking in the wrong place. Don't look at the bank account, look at their faces. Which one do you think is happier? That is what counts. Money is only money; but happiness is real wealth!