I was born and raised Catholic but gave it up sometime during high school. I found that science answered more questions than the nuns could, and those answers generally made a lot more sense. But based on what I do remember and on things I've learned since about the founding of the Church, I'd like to put forth the following explanation. Be aware, this is all my own opinion, as I'm too damned lazy to go around looking up sources right now.

The founders of the Catholic Church, in trying to imbue divinity on Christ, had to somehow simultaneously make Jesus the Son of God while at the same time BEING God. Plus they had to account for the impregnation of a virgin while avoiding the Greek and Roman problem of the gods consorting with humans. AND they had to try to reconcile the vengeful, destructive, demanding Jehovah from the Old Testament with the new view of a merciful, caring, loving God.

Somewhere along the line someone dreamed up the three aspects of God that made the most sense (to him, at least) and declared that these three aspects defined a single God with three distinct traits. Naturally, once this concept became accepted it quickly became a foundation of the Church.

And it was really cool to announce while making the Sign of the Cross, too: "in nomine Patris (head) et Filii (chest, or heart) et Spiritus (left shoulder) Sancti (right shoulder)"

The same kind of thinking went into the idea of canonizing Saints. The Roman people (after all, it was a Roman world) really liked their personalized gods and goddesses. The concept of only one God was completely alien to them. By replacing these minor gods and goddesses with the Saints the Church allowed converted Romans to have their cake and eat it, too.