Membership of Board. The membership of the Zoning Hearing Board, hereinafter referred to as the "Board," shall, upon the determination of Borough Council, consist of either three or five residents of the Borough, appointed by resolution of Council. The terms of office of a three-member Board shall be three years, and shall be so fixed that the term of office of one member shall expire each year. The terms of office of a five-member Board shall be five years and shall be so fixed that the term of office of one member shall expire each year. If a three-member Board shall be changed to a five-member Board, the members of the existing three-member Board shall continue in office until their term of office would expire as herein above provided. Council shall appoint the two additional members to the five-member Board with terms scheduled to expire promptly notify Council of any vacancies which shall occur. Appointments to fill vacancies shall be only for the unexpired portion of the term. Members of the Board shall hold no other office in the Borough. At its discretion, the Council may appoint by resolution at least one, but not more than three, residents of the Borough to serve as alternate members of the Board, pursuant to the provisions of Subsection (b) of § 903 of the Municipalities Planning Code, as added by the Act of 1988, P.L. 1329, No. 170, 53 P.S. § 10903(b).