The members of the board shall be residents of the county. The board shall be composed of 7 members appointed for terms of 3 years each. Of the members first appointed 3 shall be appointed for a term of 1 year, 2 for 2 years and 2 for 3 years. Thereafter each member shall hold office for a period of 3 years beginning January 1 next ensuing and until a successor is appointed and qualified. Each member shall file with the county clerk the constitutional oath of office.