The board, by its rules, shall provide for promotion in the fire and police departments on the basis of ascertained merit and seniority in service and examination, 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 as desire to submit themselves to examination. All promotions shall be made from the 3 having the highest rating, and where there are less than 3 names on the promotional eligible register, as originally posted, or remaining thereon after appointments have been made therefrom, appointments to fill existing vacancies shall be made from those names or name remaining on the promotional register except that promotions made in any municipality with more than 130,000 but less than 2,000,000 population may be made from the 7 members having the highest rating. The method of examination and the rules governing examinations for promotion shall be the same as provided for applicants for original appointment, except that original appointments only shall be on probation, as provided by the rules. The board shall strike off the names of candidates for promotional appointment after they have remained thereon for more than 3 years, provided there is no vacancy existing which can be filled from the promotional register.