Why is Beijing Xisanqi called "Xisanqi"? What are the allusions?

The formation and naming of Xisanqi was the result of some troops grazing horses in this area in Ming Dynasty. The military system of the Ming dynasty was to set up guards or stations in some key places to perform the duties of protecting the country and defending the country. Generally speaking, there are 5,600 guards, 1,000 households have 1 120, and 100 households have 1 12. Each command has two general flags and ten small flags. Each small flag has ten levels, and each general flag has fifty levels. It can be seen that the most basic unit in the establishment of the Ming army is also called "flag", just like today's "class". In order to resist the resurgence of Mongolian forces, the Ming Dynasty built the Great Wall on a large scale, and at the same time set up nine towns along the Great Wall, also known as "Jiubian", leading a large number of health officers and soldiers to guard the border. In order to supply the horses needed by the border garrison, many pastures and stables have been established in the mainland to graze horses, and some officers and men have been transferred to specialize in horse grazing and breeding, and even the people have undertaken the heavy task of raising horses for the government. In the Ming Dynasty, there were many horse farms and stables in Beijing, among which Huangtudian, east of Xisanqi, had a stable called Huangtudian. At that time, there were almost no villages around Huangtudian, and there were empty places full of lush grass everywhere. Officials assigned to Huangtudian stables are responsible for looking after the horses and dispersing them according to the compiled flags. Villages such as Xisanqi, Xierqi, Dongsanqi and Dongerqi were the headquarters of the official battalion of Xiaoqi in Ming Dynasty, and later evolved into villages, named after the number and location of Xiaoqi at that time. There is a village called "stable" in the east of Qinghe Town and the north bank of Qinghe today, which is the remains of stable in Qinghe in Ming Dynasty. There is also a stable village in the south of Xiaotangshan in Changping and on the north bank of Wenyu River, which belongs to Xiaotangshan stable. This can be seen from the number of stables in Beijing in the Ming Dynasty.