The three sided sort of versatility in a triangle captures themes of magic, wonder and creativity in its meaning. None of the other basic shapes offer this kind of inherent duplicity. When we turn a square on its side, the symbol meaning remains the same. Same with the circle - rolling it around, it's still a circle.
The triangle represents body mind and spirit. This connection can also extend to other abstract themes of past present and future, as well as creation, preservation and destruction. It also represent the interconnectedness of all living things conveying the idea of unifications and oneness.