Damien Evans
Up The Irons
A prequel plus 3 volumes qualifies as a series in my book... not to mention Christopher's deuterocanon.
This does not appear to me to be 3 volumes:
And Christophers edited works are either different stories set in the same world (The Silmarillion etc.) or works detailing unpublished sections, drafts, poems and other parts of his fathers writings (The History Of Middle Earth).