The Three Huan (Chinese: 三桓; pinyin: Sān Huán) refers to three noble clans, all descendants of Duke Huan of Lu, in the State of Lu, which dominated the government affairs, displacing the power of the dukes, for nearly three centuries during the Spring and Autumn period. They are the Jisun (季孫) or Ji, Mengsun (孟孫) or Meng, and Shusun (叔孫) clans.
The characters Meng, Zhong, Shu and Ji are originally ordinals used in male courtesy names to indicate a man's rank among his fathers sons. The eldest son would be Meng-something (or Bo-something), the second Zhong-something, the third Shu-something and the fourth Ji-something. For instance Confucius' courtesy name was Zhongni: "second son Hill".
As the power of the Three Huan became hereditary, the descendants of Duke Zhuang's brothers used the ordinal numbers as family names to distinguish their branches of the House of Ji. The eldest of the three was originally called Zhong since Duke Zhuang, as the eldest son, would have been "Meng"/Bo, but it was eventually changed to "Meng".
The three houses are cadet houses of the Lu ducal house, which is itself a branch of the ruling house of the Zhou Dynasty. Duke Huan of Lu (r. 711 - 694 BC) had four sons. While the oldest son by his main wife Wen Jiang became the heir, and subsequently Duke Zhuang of Lu, his other three sons - respectively Qingfu (慶父), Shuya (叔牙) and Jiyou (季友) all became important officials during the reign of Duke Zhuang, and gained substantial power in the state.
Their influence would come to undermine the power of the ducal house owing to the succession issue of Duke Zhuang. The duke, who was seriously ill, wanted his son Ziban (子般) to succeed; Shuya advocated the succession of Qingfu, but was then poisoned by Jiyou. Under Jiyou's protection, Ziban became the Duke, but was shortly murdered by Qingfu in collaboration with Duke Zhuang's wife. Jiyou was exiled from Lu, and Duke Min of Lu took the throne.
In 660 BC, Qingfu murdered Duke Min as well and sought to rule himself as the Duke of Lu; in the face of public outcry, however, he was forced to flee to the state of Ju. Jiyou then returned to Lu from Qi, his place of exile, with the younger brother of Duke Min, who then ruled the duchy as Duke Xi of Lu; while Qingfu was forced to commit suicide, Jiyou became the chief minister of Lu, a post he held for 16 years and which was secured with the backing of the powerful state of Qi.
Competition between the three families became the reason of a major skirmish during the time of Confucius. Ran Yong, Ran Qiu, and Zilu 子路 (Zhong You 仲由), three of his disciples, were ministers of Jisun, and Confucius himself was a subordinate of Viscount Huan of Ji, the chancellor of Duke Ding of Lu. Lu was almost annihilated by rebels, and Confucius allegedly took part in weakening of the three families and strengthening of the ruler's position.
At the end of the Spring and Autumn period, Duke Ai of Lu -who had no control of Lu's own army- secretly negotiated with Yue for the powerful neighbour to invade Lu and depose the Three Huan, restoring power to the duke. However the plot was discovered and Duke Ai forced to flee abroad. He was succeeded by his son Duke Dao, who continued to be a puppet of the Three Huan.