Triquetra

Triquetra (Latin tri- "three" and quetrus "cornered") originally meaning triangle 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. Originally used by Pagans it was adopted by Christians as a sign of the Blessed Trinity (Father, Son and Holy Spirit), When modern designers began to display the triquetra as a stand-alone design, it recalled the three-leafed shamrock which was similarly offered as a trinity symbol by Saint Patrick.

A very common representation of the symbol is with a circle that goes through the three interconnected loops of the Triquetra emphasising the combination of the three elements.
In Ireland, it is given to a loved one as a trinket such as a necklace or ring. It is believed to represent the three promises of a relationship such as to love, honour, and protect.

In early pagan times it is believed to represent the three stages of the Triple Goddess (Maiden, Mother, and Crone). Modern Celts use the symbol as the tripartite division of the world into the realms of Land, Sea and Sky.
I wear a triquetra as a symbol of the interconnected parts of our existence; Mind, Body, and Soul (or so my yoga teacher says) but mainly because it is a symbol of my Celtic heritage.

This one is to catch the light at the patio doors and prevent birds from flying into the glass and injuring themselves.

This is my WBC entry on the theme of "Three"