User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-170.235.245.55-20130515184452/@comment-3146930-20130515202809

They explain many times in the books that 3 is a sacred number. Basically, having three members on a quest is good luck. Also, in The Mark of Athena, they mention that if the Argo II didn't have equipment that repelled monsters, 7 powerful demigods in one place would attract monsters from all over because demigods give off a scent that monsters are attracted to.