The civil service commission shall provide for promotions within the departments under the classified civil service on the basis of ascertained merit, seniority in service, standing obtained by competitive examination, and in all cases where practicable, provide that vacancies shall be filled by promotion from among such members of the next lower rank as submit themselves for examination for promotion. The civil service commission shall certify the names of not more than three (3) applicants having the highest rating to the appointing authority for each promotion.