Causes
The Three Feudatories occupied almost half of China and caused the division of the Qing empire. Wu Sangui was granted permission by the Shunzhi Emperor to appoint and promote his own personal group of officials, as well as the privilege of choosing warhorses first before the Qing armies. Wu Sangui's forces took up several million taels of silver in military pay, taking up a third of the Qing government's revenue from taxes. Wu was also in charge of handling the Qing government's diplomatic relationships with the Dalai Lama and Tibet. Most of Wu's troops were formerly Li Zicheng and Zhang Xianzhong's forces and they were well-versed in warfare.
In Fujian province, Geng Jingzhong ruled as a tyrant over his fief, allowing his subordinates to extort food supplies and money from the common people. As for the situation in Guangdong province, Shang Kexi ruled his fief in a similar fashion to Geng Jingzhong. In total, much of the central government's state revenue and reserves were spent on the Three Feudatories and their expenses emptied almost half of the imperial treasury. When the Kangxi Emperor came to the throne, he felt that the Three Feudatories posed a great threat to his sovereignty over the empire and wanted to reduce the three lords' powers.
Read more about this topic: Revolt Of The Three Feudatories