Yet more of the same.
Of note, Set One was By Gary Gygax. Set Two was by Gary and Ernie. This one is back to just Gary.
As with the other sets, you can read the example encounter descriptions at Greyhawk Online.
The copyright date is September 21st 1977 but that’s also the date for Set Two and quite plainly wrong, given other evidence.
Judges Guild Journal, issue M, June/July states “Just released … Monster & Treasure Assortment Set One ….. The third set in the Dungeon Geomorphs series … has also just been released.” Also in the Booty List, Dragon #7 June is available.
Unfortunately, although Dragon #6 April has an ad for Sets 1 and 2, there is no mention of Set 3 (or M&TA) in any subsequent issues.
White Dwarf issue 1, June/July has lists sets 1 and 2 (and Dragon #6) for sale. Issue 2, August/September lists Set 3 (and Dragon #7). Issue 3, October/November lists M&TA Set 1.
From all this, I conclude that Dungeon Geomorphs Set 3 came out before Monster & Treasure Assortment Set 1. And they both came out around May or June. For neatness, I’m going to go with May!