In most literary pieces, the triangle (with the number three) represents perfectness, unity, and importance.
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.
Maps
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.
In Christianity
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