When a position in the classified civil service is to be filled, the appointing authority shall notify the civil service commissioner and the commission shall, as soon as possible, certify the names of three (3) or so many as there be if less than three (3) on the eligible list, to the appointing officer, provided, the said commission shall always certify the persons having the highest standing in the eligible list for the position to be filled, and each position shall be filled by one (1) of the persons certified by the said commission. All appointments shall be probationary for such periods as may be prescribed by the civil service commission. All persons not appointed shall be restored to their relative positions on the eligible list.