In philosophy, the number 3 was considered eternal because it represented an infinite cycle and repetition. In mathematics, 3 is an odd number that cannot be expressed as the product of two whole numbers, so it is considered "eternal"(infinite). In novels, the number 3 was often used as a number that represented an infinite loop and repetition. For example, in the novel 'Battle Through the Skys', the number 3 was used as a symbol to represent the many stages and experiences that the main character, Xiao Yan, had gone through. From a philosophical point of view, the number 3's infinite loop and infinite repetition made it seem eternal. In addition, the number 3 also played an important role in mathematics and the universe because it was a basic number in the ternary numbering system and was also considered the basic unit of the universe and time. Therefore, in the novel, the eternal characteristic of the number 3 could also be regarded as expressing the theme of infinite cycle and infinite repetition, as well as the essence of the infinite cycle and infinite repetition of the universe and life.