신명기 19:9의 주석

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).

כי תשמור את כל המצוה וגו'...לאהבה את ה' וללכת בדרכיו, “When you observe this entire commandment….to love the Lord your G’d and to walk in His ways, etc.” This means that the people collectively observe the laws of the Torah in all its aspects. According to Ibn Ezra the word מצוה in the singular mode does not refer to a specific commandment other than the three cities of refuge to the existing ones no additional legislation could be expected in the future.

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).

כי תשמור...כל הימים, “when you will observe for all times etc.” Nachmanides raises the question that seeing that no one (human being) knows the future, how is anyone to know when this time is ripe? He rejects Ibn Ezra (without saying why) who understands the words כל הימים to mean “without interruption,” i.e. that there will not be intervals during which not all the people keep all the commandments. Instead, Nachmanides understands the whole verse as an outline of a historical development of the Jewish people who will culminate in all of them observing all of the commandments from a feeling of love for Hashem. When this idyllic state will have been reached so that there is no longer any fear that the people will backslide from the lofty spiritual niveau, the setting aside of the additional three cities of refuge will become operative because then the territories of the last three Canaanite tribes mentioned in the prophecy to Avraham in Genesis chapter 15 will become part of the Jewish state. Only Hashem knows when that stage of development of the Jewish people will be reached, so that He will be able to fulfill that stage of expanding the boundaries of the Jewish state.