Three interlinked triangles. This design is found in Viking-age Scandinavian and Germanic sites, and is said to be associated with Odin, Valkyries, and fate or the death of warriors, although its original name and its precise meaning in period are both uncertain.
http://heraldicart.org/viking-answer-lady-svg-images-for-heralds/
Gunnvôr silfrahárr