The Three-Fold Law will not cause a spell not to manifest, nor will you experience any negative consequences of a spell that you never actually cast.
Nor does it mean that you will receive three of whatever you send out. For instance, if you curse someone and they break their finger does not mean that three of your fingers would be broken.
Nor is it meant to be a boogey man to scare people into not doing the wrong things; although, there are an awful lot of people who have fallen into this misconception.
What the Three-Fold Law is trying to tell us is that our actions have consequences and that those consequences effect us physically, emotionally, and spiritually. For instance, say you go around doing hurtful things to others out of anger....
We know scientifically that being anger all the time has a negative effect on the body and can lead to various illnesses including stomach ulcers, strokes, heart attacks, etc.
Being angry and spiteful all of the time means that we distance ourselves from others and eventually find ourselves alone and friendless. This can lead to depression.
And finally, living in an angry and hateful state can effect you spiritually because it separates you further from the Divine.
There are always consequences to our actions. They may not be immediately apparent, but over time they pile up until they do have an effect on our bodies, our emotions, and our spirits.