Then you have to take "Back to the Future" out as a trilogy and count it as "sequels" because according to the Back to the Future DVD commentaries, the producers intended the first movie to be a stand alone film and never intended to make parts 2 or 3, but they couldn't resist the popularity of the first movie and they were able to get Fox and Lloyd back.

Personally, I count it as trilogy, whether it was originally intended to be a trilogy or not, the end result works that way. The same as Matrix and Star Wars.

Come to think of it, are you sure the Godfather was intended to be a trilogy?