Rashi on Deuteronomy 19:3/רש"י על דברים י״ט:ג׳

1תכין לך הדרך THOU SHALT PREPARE THEE A WAY — “Refuge!” "Refuge!" was inscribed at each crossroad (to point the way to the nearest city of refuge) (Makkot 10b).

2ושלשת את גבול ארצך AND THOU SHALT DIVIDE THE TERRITORY OF THY LAND … INTO THREE PARTS — so that from the point where the boundary begins unto the first city of refuge there should be the same length of journey as there is from it to the second city; and that there should be the same distance from the second to the third and from the third to the other (opposite) boundary of the Land of Israel (Makkot 9b).