16 Pa. Stat. § 4512
Every position of employment, except that of superintendent of police or equivalent official, and except that of fire chief or equivalent official or chief fire inspector or equivalent official, unless filled by promotion or reinstatement, shall be filled only in the following manner: The county commissioners shall notify the commission of any vacancy in the police force or as fireman or fire inspector which is to be filled and shall request the certification of a list of eligibles. The commission shall certify for each existing vacancy, from the eligible list, the names of three persons thereon, who have received the highest average in the last preceding examination held within a period of three years next preceding the date of the request for such eligibles. The county commissioners shall thereupon, with sole reference to the merits and fitness of the candidates, make an appointment from the three names certified, unless they make objections to the commission as to one or more of the persons so certified for any of the reasons stated in the preceding section. As such subsequent vacancy occurs in the same or another position precisely the same procedure shall be followed.
16 P.S. § 4512