Why is the number three so special?

What’s so special about the number three? This is a question that has been on my mind for some time. It is a number that seems somehow intimately connected both to the structure of the universe and of the mind. Specifically, I am talking about the fact that there are three spatial dimensions (disregarding exotic extra dimensions proposed by some theories of physics), and that our languages have three persons (first-person I and we, second-person you, third-person he, she, it, they). These are two of the best examples, but there are others. Musical and written compositions are often structured in three parts: an introduction, a body, and a conclusion or recapitulation. Fairy-tales make frequent use of three, as in, say Goldilocks and the Three Bears.

But the thing which really got me thinking was the so-called Holy Trinity, consisting of father, son and holy spirit, which is part of Christian doctrine. I don’t really know what this is all about. Of course, I know that the father represents God, and the son is Jesus Christ. But what is the holy spirit? What is the meaning of this doctrine? If the number three is so special that Christianity and other religions go so far as to claim that God itself is “tripartite”, then what is it that makes them say this?