A triangle with a circle in it is used as the Sign of the Deathly Hallows in the Harry Potter universe.
“The Sign of the Deathly Hallows is a triangular mark used as a representation of the Deathly Hallows, three legendary objects that allegedly, if united, would make one the “Master of Death”. The sign is actually composed of three separate marks that, united, make up the sign. The Elder Wand is represented by the straight vertical line, the Resurrection Stone by the circle surrounding it, and finally a triangle enclosing them both to represent the Cloak of Invisibility.”
Promotional poster for Harry Potter and the Deathly Hallows