As with the second one, the first page describing how to use it, in addition to the examples, are a verbatim copy from the first M&T Assortment. And the second page is the same as M&TA2, even with the same drawing.
I’m not sure I understand why this came out so long after the first two.
Otherwise it’s much the same as the others – a bunch of random monster and treasure tables.
Honestly, the most interesting thing about all of these is actually the pictures. There are some cool ones in here including one by Bill Willingham!
Not sure if his stuff had appeared in anything before this… As far as I can tell, this is the first bit of D&D art by Willingham. Who of course, went on to do a bunch more for D&D and then went on to create the legendary Fables comic series. Love his art.
The cover is by Tramp. The other internal pictures don’t have signatures – I suspect most are by DCS, and the Harpy is most likely a Trampier.
As a lower bound, The Basic Set 2nd print, which came out in Jan 78, says that it’s forthcoming. Enworld says May. White Dwarf #8 Aug/Sep 78 has a Game Workshop ad that lists it for sale (previous issues didn’t have as extensive lists so it’s unclear if they had it before then). The Judges Guild issue 12, U, Dec 1978/Jan 1979, lists it for sale. I’m happy to go with May until any further evidence shows up.