This design represents the sign of Jonah, which Christ references in Matthew 12. Jonah is in the belly of a great fish for three days and three nights, and Christ was in the heart of the earth for three days and three nights after His death on the cross. Although the entire Trinity is involved, Christ is the part of the Trinity that takes on human flesh and dies for the sins of His people. The persons of the Trinity are represented by the triangular design with part of the triquetra highlighted at each corner, including the bottom facet within the big fish, down in the depths, symbolic of Christ's death.
Then some of the scribes and Pharisees answered him, saying, “Teacher, we wish to see a sign from you.” But he answered them, “An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of the prophet Jonah. For just as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.
—Matthew 12:38–40