Your problem is that you don't understand the difference between permutation and combination. The combination is out of order and the arrangement is orderly. If it is divided into three, the three elements will be combined in sequence, and the same three elements * * * have a factorial arrangement of three, but this problem requires disorderly combination, so factorial division of three is the order elimination method in permutation combinatorics, which can also be understood as the meaning of eliminating repeated elements, such as 1265438.