כִּֽי־תִשְׁמֹר֩ אֶת־כָּל־הַמִּצְוָ֨ה הַזֹּ֜את לַעֲשֹׂתָ֗הּ אֲשֶׁ֨ר אָנֹכִ֣י מְצַוְּךָ֮ הַיּוֹם֒ לְאַהֲבָ֞ה אֶת־יְהוָ֧ה אֱלֹהֶ֛יךָ וְלָלֶ֥כֶת בִּדְרָכָ֖יו כָּל־הַיָּמִ֑ים וְיָסַפְתָּ֨ לְךָ֥ עוֹד֙ שָׁלֹ֣שׁ עָרִ֔ים עַ֖ל הַשָּׁלֹ֥שׁ הָאֵֽלֶּה׃
就是你若謹守遵行我今日所吩咐你的這一切誡命，愛永恆者你的上帝，終身遵行祂的道，那麼你就要在這三座城之外，再為自己增添三座城。
Sifrei Devarim
(Devarim 19:9) "If you keep all of this mitzvah to do it … then you shall add for yourself, etc.": From here we see that Moses set aside three cities (of refuge) across the Jordan. When they came to Eretz Yisrael, they set aside another three, and in time to come, they are destined to set up another three. Three superadded to three — six, and another three — nine together.