Deuteronomy 19:9

If thou shalt keep all these commandments to do them, which I
command thee this day
A phrase often met with before, and signifies the putting in practice the several laws, moral, ceremonial, and judicial, which Moses was now making a repetition of, and enjoining the observance of them by a divine authority:

to love the Lord thy God;
which is the source and spring of genuine obedience to the commands of God:

and to walk ever in his ways;
noting constancy and perseverance in them; now all this is mentioned as the condition of the enlargement of their coast, which would be the case if a due and constant regard was had to the laws of God:

and then shall thou add three cities more besides these three;
three more in the land of Canaan, besides the three now ordered to be separated in it, and besides the three on the other side of Jordan; so that there would have been nine in all, if these had been ever added; but that time never came: the Jews expect the addition of these three cities in the days of the Messiah ^F25 but the Messiah is already come, and all those cities, as they were typical of him, have had their accomplishment in him the antitype of them, of which (See Gill on Numbers 35:29).