It is worthy remark, that this precept of forming three cities more, in case of the enlargement of Israel's borders, was never fulfilled as it respected Israel only. But, had it not an eye to the enlargement of the church, when both Jew and Gentile were brought into one? See Isaiah 54:2-3. And in this sense was not the precept intended to show, that the cities of refuge were as many to the Gentiles as to the Jews, and all typically referred to one and the same blessed object? Colossians 1:20.
Related Commentaries of Deuteronomy 19:8
Deuteronomy 19:8-10
8 And if the LORD thy God enlarge thy coast, as he hath sworn unto thy fathers, and give thee all the land which he promised to give unto thy fathers;
9 If thou shalt keep all these commandments to do them, which I command thee this day, to love the LORD thy God, and to walk ever in his ways; then shalt thou add three cities more for thee, beside these three:
10 That innocent blood be not shed in thy land, which the LORD thy God giveth thee for an inheritance, and so blood be upon thee.