The department shall not suspend or revoke the license of any licensee for a violation of the provisions of this chapter or a rule adopted pursuant thereto unless he has committed, within a period of one year, at least three separate violations of the provisions of this chapter or of any rule adopted pursuant thereto, and the violations have been proved by any of the following methods:
(a) A conviction for misdemeanor.
(b) A judgment in a civil suit for injunction as provided in this chapter.
(c) A finding of the department, if a hearing is held in accordance with Chapters 7 and 8 of this division.