Triquetra

Triquetra ( /traɪˈkwɛtrə/; Latin tri- "three" and quetrus "cornered") originally meant "triangle" and was used to refer to various three-cornered shapes. Nowadays, it has come to refer exclusively to a particular more complicated shape formed of three vesicae piscis, sometimes with an added circle in or around it. This has been used as a religious symbol of things and persons that are threefold.

Read more about Triquetra:  Modern Use, Geometry, Gallery of Variant Forms, Literature