TT #196 · Parashas Shoftim · Fifth Aliyah · Passuk 19:03
You shall prepare the way for yourself, and divide into three the border of your land that the Eternal your God will cause you to inherit, and it shall be for every killer to flee there.
Torah Temimah points to Makkot 9b.
TT lemma: וְשִׁלַּשְׁתָּ אֶת גְּבוּל אַרְצְךָ.
English: ‘And divide into three the border of your land’ – that they shall be distributed in three equal parts.
Derash on equal distribution (Bavli) — the language of ‘divide into three’ requires balanced coverage.
The three cities of refuge are to be spaced so that each serves roughly a third of the land.