Whenever possible, vacancies shall be filled by promotion. Promotion shall be made from among deputy sheriffs qualified by training and experience to fill the vacancies and whose length of service entitles them to consideration. The commission shall, for the purpose of certifying to the sheriff the list of deputy sheriffs eligible for promotion, rate such deputy sheriffs so qualified on the basis of their service record, experience in the work involved in the vacant position, training and qualifications for such work, seniority, and military service ratings. Seniority shall be controlling only when other factors are equal. Only the names of the three highest on the list of ratings shall be certified. The sheriff shall forthwith appoint one of the three persons so qualified.