A millennium-old tradition, recorded by Rav Sherira Gaon in 10th-century Iraq. He would always have three cooked foods on the seder plate. The egg, a product of the birds of the sky, a sign of renewal and rebirth, represented Moses, the law, the heavens, and the revelational aspects of faith. The shankbone, a product of the animals of the field, a commemoration of the original Pesaḥ sacrifice, represented Aaron, the priesthood, the earth, and the ritual aspects of faith. And the fish, representing the constant flowing nature of water, represented Miriam, prophecy, the waters, and the spiritual aspects of faith.