Why does it say "Hayu"?
Ramban: This hints that "Ashurim, Letushim and Le'umim" are not merely names of heads of nations. Onkelos translates them to be caravan camps (that go from city to city) nomads who lived in tents, and islanders (or people who live in uninhabited locations), respectively. 1 Bereishis Rabah (61:5) (and Targum Yonasan) translate them as merchants, craftsmen and heads of state, respectively.
Rashi rejects this explanation however. See Ramban. Ha'Emek Davar