Broem he already said it much more succinctly than me, but I've looked at this for a long time, so I have to type, dammit!
If you read through the various drafts of Star Wars prior to the shooting script, it's obvious that a lot of the ideas that ended up in Ep 5-6, and even 1-2, come and go as concepts in different incarnations regularly. Star Wars is a pretty lean condensation of all the best stuff, but it evidences that there was a larger canvas than what one movie could accomplish.
So, I believe he's had a story outline that had enough material and events to or ginally span three or more films, but through smart rewriting, was compacted into one great script.
When then faced with actually doing more movies, the ideas that didn't quite cut if for SW were pulled out of the drawer. Ideas, themes; not complete scripts. So, when developing ESB and ROTJ, good material came back in that made for good elaboration on existing character arcs, but ideas that were flimsy (even in the pre-76 SW drafts) also came back when momentum ran out - like the old sibling idea coming back to ROTJ. And some events that played out well in ESB eliminated themes once under consideration for ROTJ. For example again, the siblings:
I strongly believe that the sibling idea did not exist as an option post-76 until Kasdan wrote himself into a corner with Luke refusing to fight Vader. The character needed strong motivation to push him into an emotional urge to fight. Lucas and Kasdan have said as much in the past. I'm convinced that the Leia/Luke relationship was intended (in ESB dev time) to evolve into a spiritual "love" while the Leia/Han relationship was to head toward a physical "love". There's plenty of action on-screen that support that direction. That idea is kinda-sorta in ROTJ, as Luke (knowing Leia is his sister or not) chooses the solitary path of spiritual wisdom over physical needs. So while basic ideas have been there since the beginning, things change during the creative process.
It's normal really, and I don't see why this topic is such a sore point for some, are so quick to jump into conspiracy theories or think he has reason to lie about the development of the movie's storyline. You write a script that's too big to shoot. You rewrite it with the best parts. It's a big hit, and you make sequels. You revisit what you originally had, fill in the holes left by your rewrites for the original, and alter tones and themes after actors take hold of the personality of your characters and the story drives itself toward conclusion, sometimes not in the direction you originally intended.
So from the start, I say Lucas had an outline with material enough for 3 movies. He realized it before ever going before cameras on the first one, and wrote a great script that is both self-contained and open for expansion. The sequels use the ideas and themes that fell away and new ideas and themes developed from the creation of the first script.