What do the "three songs" in Hongwu period mean?

During the Hongwu period, Song Ke, Song Wangsui and Song Guang enjoyed great fame in the calligraphy field, and were called "Song San Masters". Although their calligraphy styles are different, their overall skills and elegant charm are influenced by Yuan people. Just in the Song Dynasty, Zhu Yuanzhang loved Wang Sui's calligraphy, such as "Hairpin Flower", so the calligraphy style of "Three Songs" became fashionable. On the contrary, it is the style of writing. For example, Zhan Xiyuan, who is famous for his vigorous and bold Chinese characters, was denounced by Zhu Hongwu as "sneaking around to learn from others".