The custom used to be that one stands on guard with the scroll, for it is not respectful for the Torah to remain on its own, and this would be the cantor; he would be the one who reads to the public. Two others would be positioned next to him, one to his right and one to his left. The one called to read the Torah would stand to his right, and another would stand to his left, symbolizing the Three Fathers. Despite the fact that there is no mention in early writings [Rishonim] that three are required it is, in any case, appropriate to act as it says in the Tractate of Sofrim [scribes], who came later, and were highly proficient in the customs adhered to by the People of Israel after the completion of the Talmud.