Commentary for Numbers 35:13

שש ערי מקלט [AND AS TO THE CITIES WHICH YOU SHALL GIVE] SIX CITIES OF REFUGE (THERE SHALL BE FOR YOU] — This tells us that although Moses during his life-time set aside three cities on that side of the Jordan (Deuteronomy 4:41), they did not provide refuge until the three had been chosen which Joshua assigned in the land of Canaan on the west side of the Jordan (Sifrei Bamidbar 160:2; Makkot 9b).

The three that Yehoshua placed. Rashi wishes to answer the question: Why did it write the number? Surely when it is written afterwards (v. 14), “three cities you shall give…” [Therefore] we see that there were a total of six. Rather, it was to teach that they did not provide refuge until all six were established.