Why does it mention all three Bnei Noach?
Malbim: Hashem divided settlement of the world into 70 nations, and appointed over them 70 Sarim (angels) above. The Segulah of a tree is its fruits; the leaves guard the fruits, and the rest are shells. So among people, the fruits are Elokiyim who go with Hashem. He created Eitz ha'Chayim and all people for their sake. There are animalistic people drawn after physicality. There are intermediate people who perfect their intellect and Midos according to social etiquette. Bnei Noach encompass these three. They are the root for all who come from them. The Torah stresses "Bnei Noach ha'Yotze'im Min ha'Teivah Shem Cham v'Yefes?; Sheloshah Eleh Bnei Noach", for they had to be born from Noach, for they spread over the world. The three groups come from them - Elokiyim from Shem, Amei ha'Aretz and slaves from Cham, and intermediate people from Yefes. Sometimes they dwell in the tents of Shem, and then Kena'an is a slave also to them. The roots of all three groups needed to come immediately 1 .
This should apply also to the beginning of the world, Bnei Adam ha'Rishon! Did Adam have an animalistic son? Malbim himself wrote that Shes and his son Enosh were Segulah. Kayin was Mechadesh offering a Korban to Hashem, just he sinned amidst stinginess and envy. Hevel realized that it is proper to serve Hashem from the best of one's property! (PF)
