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Star Wars (later ret-conned to Episode IV - A New Hope) was a standalone. Only after the unexpected success did Lucas say "It's set of trilogies that I must endlessly ret-con".

In the making of books it was made clear there was only to be one film originally because noone had any faith in it. Even the studio thought they had a bomb. Once he was able to make the second film, since he paid for the whole thing without studio finiancing he basically bet the farm on ESB. No one knew the potential of the sequel making as much as it did. If it had bombed there would not have been a third.

I think people misunderstood the original question. Of course the original Star Wars was a stand alone movie that most people thought would bomb. After it had unprecedented success, a sequel was all but guaranteed, especially with the main villain having survived.

I would compare this trilogy to the Matrix movies. After the successes of the first movies, two sequels were guaranteed for both, regardless to how the second movie would be received. In both cases there was such hype and demand for the sequel that was a given that it would bring in huge box office regardless if people liked it or not. For Star Wars, people loved ESB, so ROTJ was a given to be a hit as well. The second Matrix movie was seen as a let down from the original (even though I think its actually very good, just didn't live up to the impossible expectations) so the third movie pretty much bombed and has been forgotten.

They made so much money on the first one that they would have made a third even if the second one did badly.