User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-170.235.245.55-20130515184452/@comment-3285234-20130516021925

It does not need to be 3. It can be done by just one person as well i.e. Clarisse in SoM. But it's only common sense to go with two others, as the saying goes, two heads are better than one, or rather three heads are better than one. As simple as that.