Further, in this method of preaching only three statements, or the equivalent of three, are used in the theme–either from respect to the Trinity, or because a threefold cord is not easily broken, or because this method is mostly followed by Bernard, or, as I think more likely, because it is more convenient for the set time of the sermon. A preacher can follow up just so many members without tiring his hearers; and if he should mention fewer, he would occupy too little time.
Robert of Basevorn, The Form of Preaching (1322), trans. Leopold Krul O.S.B.
There you have it, folks: three points to every sermon as far back as the Middle Ages.