The problem is that most people don't truly understand what the Three-Fold Law is talking about. It doesn't mean, as an example, that if you break someone's finger then three of yours will be broken. Nor is it a threat to keep you in line and afraid of doing things for fear of punishment.
What the Three-Fold Law is saying is that our actions have consequences and that those consequences have an effect on us physically, emotionally, and spiritually - thus three-fold.
For instance, lets say you go through life angry and lashing out at those around you.
Medicine has shown that people who are constantly angry are at higher risk for heart attack, stroke, and stomach problems such as ulcers. So this would be the physical effect.
Behaving in an angry and hateful way would drive others away from you and leave you alone and isolated. This can result in depression - hence the emotional effect.
And finally, behaving in an angry and hateful way towards others creates distance between your soul and the Divine - thus the spiritual effect.
This isn't so much a matter of whether one believes or not (I happen to believe) but the observable outcome of our choices and the consequences of our action.