It's actually explained that Arceus is not part of the Creation Trio, it is referred to as "The Original One". That means that Arceus is the Pokemon that created the entire Pokemon Universe alongside the Lake Guardian Trio and The Creation Trio. It is the trio master of the two trios and not exactly part of the trio itself. The Creation Trio is a trio that can control of their own dimesions, while Arceus just manages the universe at the Hall of Origin. Arceus also created the members of the Creation Trio, as explained in the HGSS Sinjoh Event where you received a LV 1 Dialga, Palkia, or Giratina.