Rashi on Numbers 35:14/רש"י על במדבר ל״ה:י״ד

1את שלש הערים וגו׳ THE THREE CITIES [YE SHALL GIVE ON THIS SIDE OF THE JORDAN etc.] — Although in the land of Canaan there were nine tribes and here (on the east side of the Jordan) there were only two and a half, it (Scripture) gives them (the latter) the same number of cities of refuge, because in Gilead (on the east side) murderers were more numerous, as it is written, (Hosea 6:8): “Gilead is a city of them that work iniquity, it is covered with footprints of blood” (Makkot 9b; cf. Sifrei Bamidbar 160:2).