The Third Edition is marked as such. The 1st and 2nd Edition don't contain any information not in the third with a couple exceptions: The Atlas Addendum was an Addendum to the 1st edition atlas that contains a few bits of info on artifacts that have not yet been reproduced elsewhere. There's also a global world map showing essence flows from that same box set.
All in all it's a minimal amount of material and neither of those editions are necessary if you have the third.
You're also missing a few Loremaster and Navigators in SWMA3 or 4 that were in 1 or 2. The Loremaster, Navigator and Essaence Spell Lists. ( the lists can be found on Terry's website though IIRC )
But I agree the missing content is minimal ( IIRC I made a comparison that is probably hidden somewhere in the archives ).
The First Edition is the Shadow World Boxed Set, with the World Book and the Inhabitant Book
Master Atlas 2 is a reprint in a book of the two books that were in the SW Boxed Set, the cover is the one from the Boxed set ( and is also a cover used in a book from Alan Dean Foster
As several other covers on SW and SM adventures ). It's written on the cover that it's the second edition. Main color is blue.
Master Atlas 3 adds stuff from the Atlas Addendum along with lots of new things and update the world according to all the events that happened in the cannon adventures. Mine is Spiral bound, and I don't think there's been an edition that wasn't spiral bound. It's written on the cover that it's the Third edition. Main color is red/brown.
Master Atlas 4 updates the content of the Master Atlas 3 and add a few more things. ( like the RMSS/FRP stats ). Main color is Purplish.