Almost every culture has some significant symbolism invested into the triangle.
To the Druids it was the breakdown of their holiest number 9 (being three three's)
In Japan a Shinto ritual which waves a paper wand in the shape of a triangle is thought to bring harmony to a home and keep spirits away from the doorway.
As stated previously, Neo Paganism has adopted it to represent the three faces of the Goddess.
The cult in Greece which started in essence, modern mathematics held it second only to the pentagram.
It's one of mans oldest symbols because it's one of mans oldest epiphanies, the connection of mind body and spirit and the triple nature of the universe.
All important things come in three's.
I'm curious as to what sparked the question