Every position, unless filled by promotion, reinstatement or reduction, shall be filled only in the manner specified in this section. The appointing officer shall notify the firemen's civil service commission of any vacancy in a position which he desires to fill, and shall request the certification of eligibles. The commission shall forthwith certify, from the eligible list, the names of the three individuals thereon who received the highest averages at preceding competitive examinations held under the civil service provisions of this article within a period of three years next preceding the date of the prospective appointment. The appointing officer shall, thereupon, with sole reference to the relative merit and fitness of the candidates, make an appointment from the three names so certified: Provided, That should he make objection, to the commission, to one or more of these individuals, for any of the reasons stated in section nineteen of this article, and should such objection be sustained by the commission, after a public hearing along the lines of the hearing provided for in section nineteen, if any such hearing is requested, the commission shall thereupon strike the name of any such individual from the eligible list, and certify the next highest name for each individual so stricken. As each subsequent vacancy occurs, in the same or another position, precisely the same procedure shall be followed: Provided, however, That after any name has been three times rejected for the same or another position in favor of a name or names below it on the same list, the said name shall be stricken from the list. When there are a number of positions of the same kind to be filled at the same time, each appointment shall, nevertheless, be made separately and in accordance with the foregoing provisions. When an appointment is made under the provisions of this section it shall be, in the first instance, for the probationary period of six months, as provided in section sixteen of this article: Provided further, That in the event any position as an electrician or mechanic is to be filled in any paid fire department, then the examinations to be given to applicants for either position shall be so drawn as to test only the qualifications of such applicants in regard to their ability as electricians or mechanics, such examinations to be special examinations.