The triquetra is a three-pointed symbol that has been used in ancient Nordic, Celtic, German, and Japanese cultures. It has been used in both pagan and Christian religious imagery as well, symbolizing the Holy Trinity in the latter. It is sometimes interlaced within a circle or combined/interlaced with additional triquetras to form different geometric shapes like crosses. Celtic knots, trikelions, and the three-hare symbol are all related.
In the first two seasons of Dark, the triquetra seems to symbolize the caves' closed time loops with each year being thirty-three years apart, with the past affecting the future and the future influencing the past. In the third season, it comes to symbolize the three different worlds as well.