7-3-4258. Civil service examinations. Said commission shall, on the first Monday of April and October of each year or more often if it shall be deemed necessary, under such rules as may be prescribed by the council, hold examinations for the purpose of determining the qualifications of applicants for positions, which examination shall be practical and shall fairly test the fitness of the persons examined to discharge the duties of the position to which they seek to be appointed. Such commission shall, as soon as possible after such examination, certify to the council double the number of persons necessary to fill vacancies, who according to the records have the highest standing for the position they seek to fill as a result of such examination, and all vacancies which occur that come under the civil service prior to the date of the next regular examination shall be filled from said list so certified; provided, however, that should the list for any cause be reduced to less than three for any division, then the council or the head of the proper department may temporarily fill a vacancy, but not to exceed 30 days.