The Town Board may appoint two alternate members to the Planning Board, which the Town Board shall designate as the first alternate and second alternate. The first alternate member, so appointed, shall serve for a term expiring three years from the end of the official year in which they are appointed, and his successors shall serve for a term of three years from and after the expiration of the terms of their predecessors in office. The second alternate member, so appointed, shall serve for a term expiring two years from the end of the official year in which he is appointed, and his successors shall serve for a term of three years from and after the expiration of the terms of their predecessors in office. Should a vacancy occur for reason other than the natural expiration of a term, the Town Board may fill the vacancy for the unexpired term.