On and after the effective date of this article, every position of correctional officer, unless filled by promotion, reinstatement or reduction, shall be filled only in the manner specified in this section. The appointing sheriff shall notify the civil service commission of any vacancy in a position of correctional officer 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 persons thereon who received the highest averages at preceding competitive examinations held under the provisions of this article within a period of three years next preceding the date of the prospective appointment. The appointing sheriff 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 persons for any of the reasons stated in section ten 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 said section ten, if any such hearing is requested, the commission shall thereupon strike the name of that person from the eligible list, and certify the next highest name for each person 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 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 twelve months, as provided in section seven of this article.