The Civil Service Commission shall, by its rules and regulations, provide for promotions in the classified civil service on the basis of merit, seniority in service and examinations, and shall provide in all cases, where it is practicable, that vacancies shall be filled by promotion. All examinations for promotion shall be competitive among such members of the next lower rank who desire to submit themselves to such examination, and it shall be the duty of the commission to submit to the appointing power the names of the 3 highest eligibles for each promotion. The appointing authority shall then appoint one of the 3 certified persons to such vacant position. In fixing said rating, a uniform allowance of credits, to be stated at the time of the announcement of said examination, shall be made for each year of past service. The method of examination, the rules and regulations governing the same and the method of certifying, shall be the same as provided for applicants for original appointment.