I've been trying to dig up exactly what the different sound formats of the original theatrical releases were, and here's the latest rundown to the best of my knowledge. The first two digit number refers to the Dolby Format Codes. It appears that there were differences in the audio content on each format for all the movies, with perhaps the exception of "Return of the Jedi". At least I have not seen any documented differences for ROTJ.

"Star Wars" 1977

42-70mm 6-Track (Baby Boom - see note 1)
04-35mm Dolby Stereo
02-35mm Monaural (see note 2)

"The Empire Strikes Back" 1980

42-70mm 6-Track (Baby Boom)
04-35mm Dolby Stereo

"Return of the Jedi" 1983

42-70mm 6-Track (Baby Boom)
04-35mm Dolby Stereo

Note 1. The original 70mm 6 track consisted of tracks 1-5 being used in order from left to right behind the screen for full range reproduction, and track 6 being reproduced by multiple speakers around the rear of the theater for surround effects. Dolby Labs came up with an idea to use tracks 2 and 4 to extend bass response rather than using them for full range reproduction, leaving tracks 1, 3, and 5 as Left, Center, and Right respectively, and of course track 6 for surround. This is what is known as "Baby Boom". "Star Wars" was one of, if not the first movie, to use the "Baby Boom" system. The later "Stereo Surround" system where the upper frequencies of tracks 2 and 4 were combined with the bass from channel 6 to create "Split Surrounds" was never used for any of the Star Wars movies to the best of my knowledge.

Note 2. It appears that prints using the monaural soundtrack were not put in circulation until June 1977, which implies that the theaters showing "Star Wars" prior to this time that were not equipped for stereo reproduction were showing Dolby Stereo prints (which were mono compatible).

Dolby Sound System for Star Wars This period article gives some good details of the production of the soundtrack for "Star Wars". Most of the details pertinent to "Star Wars" itself are in the last section on Page 2.

Edit - Added Dolby Format Codes. Specified original theatrical releases.