http://www.erikstormtrooper.com/banthatracks08.pdf
And here's the relevant text:
SW: At one point there were going to be twelve Star Wars films.
GL: I cut that number down to nine because the other three were tangential to the saga. Star Wars was the fourth story in the saga and was to have been called "Star Wars, Episode Four: A New Hope." But I decided people wouldn't understand the numbering system so we dropped it. For Empire, though, we're putting back the number and will call it "Episode Five: The Empire Strikes Back." After the third film in this trilogy we'll go back and make the first trilogy, which deals with the young Ben Kenobi and the young Darth Vader.
SW: What is the third trilogy about?
GL: It deals with the character that survives Star Wars III and his adventures.