As has been mentioned, the Three-Fold Law is basically saying that our choices of action have consequences, and that we are responsible for the consequences of our actions.
Some believe that the Three-Fold Law means that if you do something bad then something three times worse will happen to you. Not only is this a misunderstanding of the Three-Fold Law but it suggests that we should do good because we're afraid of punishment when we should be acting rightly because it is the proper thing to do.
When the Three-Fold Law talks about things returning three times over what it is really saying is that the consequences of our actions come back to effect us physically, emotionally, and spiritually.
As an example, if you go through life angry and striking out at those around you there will be consequences.
We know that anger and stress can have a negative effect on you physically. It can cause gastric problems. It can make you more prone to heart attack and/or stroke.
If you're always acting hatefully to others you will eventually be left without friends. This can lead to isolation and depression., thus the emotional impact.
And spiritually, such actions lead to separation from the Divine.
The Three-Fold Law is nowhere as simplistic as many make it out to be. Think of it as the natural result of your own actions and how those impact you.