Expanding on what Fang Zei started, here are the Library of Congress record pages for the Original Trilogy - Opening Day. There's other info, so check the full link, but i've copied out the 'Acquisition Source' these are the archived 'Registry' versions.
1977 - Star Wars
http://lccn.loc.gov/96512247
Acquisition Source: Received: 7/8/83; viewing copies (copy 1 and 2); purchase, purchase order no. 0124319; LC Purchase Collection.
Received: 11/88; viewing copy (copy 3); purchase; LC Purchase Collection.
Received: ca. 1983 or 1984; viewing copy; purchase; LC Purchase Collection.
Received: 3/28/78; ref print; copyright deposit--advanced selection; Copyright Collection.
Received: 6/19/79; ref print (FGD 5534-5539); copyright deposit--no agreement; Copyright Collection.
I haven't done an exhaustive search yet so this is just the first 'Star Wars' film version which i've stumbled upon. As mentioned by LexX, the Library of Congress might not have the 'First Published' version of Star Wars the 70mm. From this record the 'First Published' is 1978, possibly a 35mm print from the theater run. Then another print from 79, and some of the laserdiscs and CED versions. But before getting excited they have a large database, others need to poke around for records.
1980 - Empire Strikes Back
http://lccn.loc.gov/96512208
Acquisition Source: Received: 11/1988; viewing copy; purchase; LC Purchase Collection.
Received: 10/9/1980; viewing print; copyright deposit--no agreement; Copyright Collection.
This 1980's version again probably submitted after the theater run, is 35mm and contains the additional scenes not found in the opening day 70mm version. (see pg.333 of 'the making of ESB' book) Mr. Leggett mentioned that although the film was selected to the Archives, they have not asked CBS/Fox/Lucasfilm yet for the new print. We'll have to wait until the summer and ask about what they received.
1983 - Return of the Jedi
http://lccn.loc.gov/96501520
Acquisition Source: Received: 11/7/91; viewing copy; purchase, purchase order no. 172525; LC Purchase Collection.
Received: 6/15/83; ref print; copyright deposit--no agreement; Copyright Collection.
A few months after release, again probably a theater copy. Don't know enough about the last minute production to know if there might be changes from the 70mm for RotJ. Was there a 70mm RotJ?