"and you inherit them and you dwell in their cities and in their houses": Because it is written "and you inherit them and you dwell in their cities," I might think that you are not permitted to add (to the existing settlement); it is, therefore, written (Ibid. 12:29) "and you dwell in their land" — Wherever you wish to build, build.
(Devarim 19:2) "Three cities": of average size, not very small and not very large.