The Law of Three is a reminder that every action
spawns a reaction, and any energy you send out will naturally come back to
you in some form or another. People have different interpretations of what
this means and may or may not take it literally. For example, if you cast
a spell that causes someone to get sick, does it really mean that you will
get sick three times as a result? Personally, I would not expect that to
happen. On the other hand, some pagans might say yes. This is all a
matter of learning and personal experience.
From my perspective, the Threefold Law is very important because it
helps me focus before a ritual. It encourages me to think about the goal
and whether it is truly positive or negative, who it can help and who it
may harm-- and I have the freedom to adjust the ritual or toss it out. The
Law can also help in ordinary settings-- If you're struggling with a major
decision, particularly one that affects someone else, you might want to ask
yourself which option has the best outcome for everyone involved, and the
environment.
Regardless of how you interpret it, I think the Threefold Law has
some value, but the most effective way to understand it is to
examine your own experiences.