The commission shall consist of 3 members. One member of the commission may also be on the planning board. The members of the commission shall be appointed by the selectmen. Alternate members may be appointed in a like manner and when the alternate serves in the absence or disqualification of a regular member, the alternate shall have full voting powers. When a commission is first established, terms of the members shall be for one, 2, or 3 years, and so arranged that the terms of approximately 1/3 of the members will expire each year, and their successors shall be appointed for terms of 3 years each. Any member of a commission so appointed may, after a public hearing, if requested, be removed for cause by the appointing authority. A vacancy occurring otherwise than by expiration of a term shall be filled for the unexpired term in the same manner as an original appointment. Members of a conservation commission shall be residents of the town. Members of the conservation commission also may serve on other municipal boards and commissions.