At the annual town or district meeting when such board is established, there shall be chosen, by ballot and by major vote, 3 sewer commissioners, to hold office for 3 years, 2 years, and one year, respectively, and thereafter, at every annual meeting, one commissioner shall be so chosen to hold office for 3 years; provided, that such election shall be by plurality vote in towns or districts which, under existing laws, elect officers in that manner.