The reason why the Kangxi Emperor was able to easily defeat the powerful three feudatories was not accidental, but was based on many reasons. First of all, the Kangxi Emperor's use of military strategy can be called brilliant. He adopted a strategy of dividing and dealing with each of the feudal kings, defeating them individually to prevent them from forming a joint force. He quickly revoked Shang Kexi and Geng Jingzhong's feudal slashing and sent envoys to persuade them to surrender, successfully breaking their will to resist. In addition, the Kangxi Emperor also took corresponding measures according to the different circumstances of each vassal king, such as taking a swift military attack on Wu Sangui, thus quickly weakening his power. 
Secondly, the contradictions within the three feudatories were also an important reason why Emperor Kangxi was able to win easily. Although Wu Sangui was strong, Shang Kexi and Geng Jingzhong were relatively weak, and their anti-Qing actions were more out of a lively mentality than an iron determination to fight the Qing Dynasty to the end. Even later in the war, Shang Kexi sent troops to attack Wu Sangui, and this internal contradiction led to a drain of strength, allowing the Kangxi Emperor to easily defeat the three feudatories. 
Moreover, Wu Sangui's innate conditions for raising troops against the Qing Dynasty were also an important factor in Emperor Kangxi's easy victory. Although the southwestern region occupied by Wu Sangui was difficult to terrain, its resources and economic strength could not compete with the Jiangnan region controlled by the Qing Dynasty. In addition, the three feudatories were not subordinate to each other, unable to form a joint force, and even attacked each other, consuming each other's strength. In addition, although the combat power of the Eight Banners has declined, it still has a strong deterrent power, while the southern soldiers are not tolerant of the severe cold in the north, so the northern expedition is rarely successful. In the end, Wu Sangui was already over sixty years old when he raised his army, and he had long lost the ambition of his youth. 
To sum up, the reason why the Kangxi Emperor was able to easily defeat the powerful three feudatories was mainly due to his proper use of military strategy, the contradictions within the three feudatories, and the innate conditions for Wu Sangui to raise an army against the Qing Dynasty. The combination of these factors enabled the Kangxi Emperor to successfully quell the rebellion of the three feudatories, further consolidating his dominance.