(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.
(דברים יט ט) כִּי תִשְׁמֹר אֶת כָּל הַמִּצְוָה הַזֹּאת לַעֲשֹׂתָהּ... וְיָסַפְתָּ לְךָ עוֹד שָׁלֹשׁ עָרִים עַל הַשָּׁלֹשׁ הָאֵלֶּה, מִכָּן אַתָּה אוֹמֵר: שָׁלֹשׁ עָרִים הִפְרִישׁ מֹשֶׁה בְּעֵבֶר הַיַּרְדֵּן, וּכְשֶׁבָּאוּ לָאָרֶץ הִפְרִישׁוּ עוֹד שָׁלֹשׁ וְלֶעָתִיד לָבוֹא מַפְרִישִׁים עוֹד שָׁלֹשׁ; שָׁלֹשׁ עַל שָׁלֹשׁ – הֲרֵי שֵׁשׁ, וְעוֹד שָׁלֹשׁ – הֲרֵי תֵּשַׁע.
(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.
רַבִּי נְהוֹרַי אוֹמֵר: שָׁלֹשׁ עַל שָׁלֹשׁ, וְעוֹד – הֲרֵי תֵּשַׁע, עַל הַשָּׁלֹשׁ – הֲרֵי שְׁתֵּים עֶשְׂרֵה.
R. Nehorai says: These three, added to three; "then you shall add for yourself" — nine; "another three" — twelve.
רַבִּי שָׁאוּל אוֹמֵר: שָׁלֹשׁ עַל שָׁלֹשׁ, וְעוֹד – הֲרֵי תֵּשַׁע, עַל הַשָּׁלֹשׁ – הֲרֵי שְׁתֵּים עֶשְׂרֵה, הָאֵלֶּה – הֲרֵי חֲמֵשׁ עֶשְׂרֵה.
Abba Shaul says: Three added to three, "another three" — nine; "on the three" — twelve; "these" — fifteen. ...