When the Torah commands the establishment of cities of refuge in Eretz Yisra'el to protect accidental murderers from vengeful relatives it states that the three cities must be evenly placed (וְשִׁלַּשְׁתָּ) to maximize the population's access to them. A couple sentences later it says that when the borders of the land are expanded, three more cities should be added to this set of three. When the borders did expand, were the original three repositioned to maximize their centralization in the larger area?
When the Jews conquered Canaan, they were effectively fulfilling God's promise as described in Devarim 7:1 by the destruction of the seven nations listed there.
However, in His promise to Avraham, ten nations are specified (Bereshis 16:19-21). Traditionally, the extra three nations' demise are believed to be referring not to the time of Yehoshua's conquering of the land, but to the Messianic era.
Ramban believes that this is what is meant by "ואם ירחיב יהוה אלהיך את גבלך כאשר נשבע לאבתיך", that is, the conquering of those three nations in the times of Mashiach, or in Ramban's language, "יורה על כל הימים העתידים לבוא".
As Ramban is not entirely clear there with his intention, I'll include Abarbanel's explanation of his interpretation, which is much clearer (in my opinion):