The Origin of the Jews of Yathreb

How did these Jews of Yathreb get divided up into three tribes when, if the story is correct, they're all coming from one area and settling in Yathreb? It doesn't make sense for them to be three separate tribes. In other wars, there's a civil war taking place between the Arabs and between the Jews. The theory that I have in Allah subhanahu wa ta'ala knows best is that there's an element of truth in more than one of these stories.