idiom · People from various social backgrounds or trades, often referring to a diverse or disreputable mix of individuals.

旧时代的火车站经常聚集着三教九流的人物。

idiom · The Three Religions (Confucianism, Buddhism, Taoism) and the Nine Schools of thought in ancient China.

三教九流是中国古代对各种宗教和学术流派的合称。