John Gill Commentary Deuteronomy 19:7

"Wherefore I command thee, saying, Thou shalt set apart three cities for thee." — Deuteronomy 19:7 (ASV)

Wherefore I command you, saying, you shall separate three
cities for you .
] This was to be done immediately, as soon as they were settled in the land of Canaan, and established in the possession of it, the inhabitants being cut off, or driven out, or however subdued.