Section 69A. Any town establishing a water supply or water distributing system under authority of section thirty-nine A of chapter forty may establish a board of three water commissioners or authorize its selectmen to act as such. Such commissioners shall, in the first instance, be elected by ballot to hold office, one until the expiration of three years, one until the expiration of two years, and one until the expiration of one year, from the next succeeding annual town meeting; and at the annual town meeting held on the day on which the shortest of such terms expires, and at each annual town meeting thereafter, one such commissioner shall be elected by ballot for the term of three years. A majority of said commissioners shall constitute a quorum for the transaction of business. After the election of a board of water commissioners under authority of this section, any vacancy occurring in said board from any cause may be filled for the remainder of the unexpired term by said town at any legal town meeting called for the purpose. Any such vacancy may be filled temporarily in the manner provided by section eleven, and the person so appointed shall perform the duties of the office until the next annual meeting of said town or until another person is qualified. Nothing herein contained shall prevent action under section twenty-one or sixty-nine.