All such trusts shall be administered by a board of 3 trustees, unless a town at an annual or special town meeting votes that such trusts shall be administered by a board of 5 trustees. In towns with a board of 3 trustees, one trustee shall be elected by a ballot at each annual town meeting for a term of 3 years. In towns with a board of 5 trustees the 2 additional trustees shall be appointed initially by the selectmen, one for one year and one for 2 years. Thereafter all trustees shall be elected by ballot at the annual town meeting to replace those whose terms expire. The term of each trustee shall be 3 years. Vacancies shall be filled by the selectmen for the remainder of the term. The board may recommend to the appointing authority the names of no more than 2 persons who may serve as alternate members on the board. The alternate members shall be appointed to one-year terms. In cities said trustees shall be chosen and hold their office for such term as shall be provided for by city ordinance. Trustees shall organize by electing one of their number bookkeeper, who shall keep the records and books for the trustees, and shall require a voucher before making any disbursement of funds from said trusts.