Any such company may adopt bylaws for the conduct of its business not repugnant to law or to its charter, and shall therein provide for the election of a minimum of three (3) directors. The bylaws may provide for the division of its board of directors into two (2), three (3) or four (4) classes and the election thereof at its annual meetings in such manner as that the members of one (1) class only shall retire and their successors be chosen each year. Vacancies in any such class may be filled by election by the board for the unexpired term.