@super_etecoon
A New Hope was not expected to be the first movie in a trilogy. The resulting success gave George Lucas that opportunity. A New Hope was a self contained story. The movie had a true story arc. He knew he probably would only get one shot to share a snapshot of his vision. So he picked out the best part, and then fleshed that out.
The second trilogy was obviously a planned trilogy. However it was probably only a trilogy due to the fact that the first three movies were a trilogy so the mold was set. However if you compare the Phantom Menace and A New Hope. You will plainly see the lead effect for the next film.
Frankly thanks to A New Hope I don't think we have gotten a decent trilogy on the big screen yet. They are technically trilogies, but the story arcs in both sets are mangled by the stand alone story that was A New Hope.
Lord of The Rings wasn't even a mediocre trilogy. The story was entirely linear, and the books could never translate into a decent theatrical experience. No plodding narrative should ever be converted into a movie or a string of movies. Outside of literature we haven't gotten a single working trilogy yet.
I think this should be a wonderful experiment to watch. Nobody has actually succeeded in either film or gaming. Here we have the outright intent to do a trilogy, and we probably have a story that will not succumb to a lost in translation experience.
I doubt even Mass Effect qualifies. I suppose we will have to see if the story has enough cross structure, but initially the first game seems to have been very self contained. I have the feeling that Bioware played it safe.