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。
Rashi
ויספת לך עוד שלש THEN THOU SHALT ADD THREE [CITIES] MORE FOR THEE — Thus you have nine altogether: the three on the east side of the Jordan, the three in the land of Canaan and three more in the future to come when God will enlarge thy territory (Sifrei Devarim 185:1).
Rabbeinu Bahya
ויספת לך עוד שלש ערים, “then you shall add three more cities to these three.” A third group of three cities of refuge in addition to the two groups of three already established (in the territory which used to belong to seven of the ten tribes).