I believe that the triad is the strongest formation of magic. The triangle is the strongest shape there is. A circle has flaws. It can be penetrated while the rest of the circle does not realize. With a triangle, if one corner is severed, the other two know.
I beleive the world is on a sensitive balance of good and evil. If one should overcome the other, then the world will regret it.
For instance, if there is no evil in the world, the heros and do-gooders of our time shall become lazy. Eventually, they will grow to believe they deserve to have recognition for their long and hard battles. This leads to greed and not long after, the world is ruled by evil. Not long after, an army of good will rise up to destroy the evil. In this way, the cycle will begin and the balance is ruined, there never being happiness.