The Civil Service Commission shall be composed of three electors to be appointed by the Mayor with the approval of the Council. The members of the existing Commission shall continue in office for the terms for which they were severally appointed. The term of each commissioner thereafter appointed shall be for six years; but appointments for full terms or to fill vacancies shall be so made that on the 31st day of March in each even numbered year, the term of one commissioner, and one only, shall end.