Lecture quote:

"The first Arcanum, related to the projective masculine force, unity, projects, but it is balanced and equilibrated by the second factor, or the second Arcanum, which is duality.  So then we have these two forces in a dual relationship, but for creation to actually occur, for the world of Daath to actually blossom and manifest existence, manifest creation, there has to be a third factor.  

This Law of Three is that profound and deep equilibrium between forces.  We have this symbolized in many of the structures of the Tree of Life.  We have three columns upon which hang three triangles, and each triangle is made of three spheres.  The Law of Three is the very structure of our Tree of Life, both within us and outside of us.  

The Law of Three becomes something very foundational for our understanding of the Kabbalah, as it does provide the very foundation of the Tree itself.  More than that, the Law of Three provides a vital foundation for the work that we have to perform in order to self-realize all the Sephiroth.  The self-realization of the Sephiroth means perfect comprehension of all the laws that manage the Sephiroth.  So when we look at this Law of Three, we are actually looking at something that we have to embody.  We have to learn how to work with the three forces of creation.  

In the symbolism of many religions, we have at the root of creation certain forces or factors.  Water is a very common, universal symbol related to creation.  We also find air, or Spirit; we also find fire.  These three root elements are also symbolized in the Zohar and in the Sepher Yetzirah, which is an ancient book of Kabbalah.  And these three forces, which underlie all of manifested creation, are called the Three Mother Letters.  They are Aleph, Mem, and Shin.