Here is what St. Thomas Aquinas says
Through a misunderstanding, the Jews associated three great personages with the coming of Christ: 1) Christ himself, 2) Elijah, 3) and some other person, the greatest of the prophets, about whom Deuteronomy (18:15) says: “The Lord your God will raise up a prophet for you.” And although this greatest of the prophets is in fact none other than Christ, according to the Jews he is someone other than Christ. And so they do not ask simply whether he is a prophet, but whether he is that “greatest of the prophets.” And this is clear from the order of their questions. For they first ask whether he is the Messiah; secondly, whether he is Elijah; thirdly, whether he is that prophet.