Rashi on Numbers 1:52/רש"י על במדבר א׳:נ״ב

1ואיש על דגלו [AND THE CHILDREN OF ISRAEL SHALL ENCAMP, EVERY MAN BY HIS OWN CAMP] AND EVERY MAN BY HIS OWN STANDARD, just as the order of the banners is given in this Book (Numbers) — three tribes for every banner.