Three of the members first appointed shall be designated to serve the terms of one, two, and three years, respectively, from the date of their appointments and two shall be designated to serve for terms of four years from the date of their appointments. If and when the membership of the agency is increased to seven, one of the additional members shall be appointed to a term, or unexpired portion thereof, which is concurrent with the term then held by the member originally appointed for a term of three years or by his successor, and the other additional member shall be appointed to a term, or unexpired portion thereof, which is concurrent with the term then held by the member originally appointed for a term of two years or by his successor. Their successors shall be appointed for four-year terms. Vacancies occurring during a term shall be filled for the unexpired term. A member shall hold office until his successor has been appointed and has qualified.