"as He swore to your forefathers" — all in the merit of the forefathers.
"and He give you all the land that He swore to give to your forefathers": These are (the cities of) the Keni, the Kenizi, and the Kadmoni (across the Jordan, where three cities of refuge were established).
(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.