An athame is used as an energy conduit.  It focuses, channels, and directs energy where you want it to go.  It is not used as a traditional knife, to cut things on the physical plane, but to direct and cut on the etheric plane.

The triquetra is a Celtic knot with a triangular design symbolizing all trinities including the Triple Goddess (Maiden, Mother & Crone); the eternal cycle of life, death, and rebirth, and the realms of Air, Earth and Sea which, to the ancient Celts, represented the entire Universe. To Christians, the triquetra has been used to symbolize the Holy Trinity (Father, Son & Holy Ghost) In fact, many cultures and religions have considered the number three to be holy or divine for millennia. A symbol of three interlocking circles looking somewhat like a three-leaf clover without the stem has been found on religious statues in India that were made 5000 years ago.