Section 12. In cities which accepted chapter three hundred and thirty-two of the acts of nineteen hundred and one, the term of office of the city clerk shall be three years from the date of his election or appointment, except that in cities required to elect a city clerk in December his term shall be three years from the beginning of the next municipal year. In either case the clerk shall serve until the qualification of his successor unless sooner removed; and a person appointed to fill a vacancy in the office of city clerk shall hold the office until the end of the unexpired term of his predecessor.