A triangle is a two-dimensional shape with three angles.
In most literary pieces, the triangle (with the number three) represents perfectness, unity, and importance. It is the strongest unit. When a group/item in literature moves from three, the triangle, to four, it foreshadows bad things and destruction. In literature, most duos of people/characters are traveling towards three. However, when they gain an extra member, a fourth, a dark turn is about to take place. In some extreme cases, a member of the group dies, so that the perfect three can happen again.
A triangle represents the fourth letter of the Greek Alphabet called Delta. It is from the Phoenician Dalet, which is also a triangle. In Physics the Delta is used to represent change.
In most older maps a triangle is drawn in place of a mountain or mountain range, in modern maps this symbol is occasionally used for mountain in keys or legends.
The equilateral, having three identical angles, represents the Holy Trinity of the Father, Son and Holy Spirit. This also goes along with Christianity's extensive symbolism of three. When a circle is added to the center of the equilateral triangle it is considered to represent the eternity with in the Trinity. It is believed that this is the inspiration of the Deathly Hallows Symbol in the Harry Potter series. In the seventh book it is explained that the triangle represents Harry Potter's Invisibility Cloak