Members of the hearings board shall be appointed for a term of six years and until their successors are appointed and have qualified. In case of a vacancy, it shall be filled by appointment by the governor for the unexpired portion of the term in which said vacancy occurs: PROVIDED, That the terms of the first three members of the hearings board shall be staggered so that one member shall be appointed to serve until July 1, 1972, one member until July 1, 1974, and one member until July 1, 1976.