The third booklet is kind of like the DM’s guide and probably the weirdest of them all.
In some ways, it seems the least developed and has the most references to use of the Chainmail rules.
The sample dungeon is bizarre – all sorts of tricks and traps and a mapping nightmare. There’s also a weird section about keeping a dungeon fresh – after a party has cleared out a level, do things like move walls, add rooms, etc. And then a section on wandering monsters.
The example referee/player session shows an example of providing almost inscrutable dungeon descriptions to the poor players.
The next section is about the wilderness and things get stranger. It suggests using “Outdoor Survival” – an Avalon Hill game. And the wandering monster lists have all sorts of odd creatures that appear nowhere else in the whole game, like Orluks, Banths, Calots, and even Martians! (Oh, those other creatures are all in John Carter of Mars).
We then have a section about building and manning castles and dealing with villagers and land combat of large armies.
Following that there are quite large sections on aerial combat and naval combat. This was all a bit fresh to me – it’s not something we ever did in our D&D sessions and I’m kind of surprised to see a similar section with about the same amount of detail in the 1st edition AD&D DM’s guide now that I look at it.
Finally there’s a section on sea monsters where we finally get the details of the dragon turtle, amongst others. It’s unclear why these two pages of monsters weren’t included in the Monster booklet.
Oh and if you have a 5th or earlier printing, you get the Nazgul and Ent labels on the pictures.