I think you're right, the original idea was 3 sets of 3 trilogies, and the first 3 released (Episodes 4,5,6) were intended as the middle trilogy; however, Lucas didn't know if he'd be able to make the first trilogy (Episodes 1,2,3) in the 70s, which is why the 'Episode 4/5/6' titiles didn't appear until the re-release in the 90s.
Lucas has since retracted earlier statements once Episodes 1,2,3 were made. Having seen how the stroyline panned out from Episodes 1-6 ( the rise and fall of Darth Vader) and the fact the the extended universe (ie all the novels) are accepted as part of continuity, Lucas now has no plans to make a further trilogy of films.
He's supposedly content to leave the films as they are, as they follow a course to a natural end, with Vader's death. As far as he's concerned, the novels do an adequate job continuing the series. instead he's decided to focus his energies on the Clone Wars era - the gap between episodes 2 and 3, which so far we have had animated TV shows and an animated movie devoted to them; and if i'm right a live action TV series to follow.
Of course, in about 10 years the original trilogy will be rereleased, with Jar Jar Binks digitally added to key scenes. It's a little known fact that Jar Jar was the real pilot who destroyed the second Death Star.
Coo LP Coo