My apologies if I've overlooked something but M&M III seems to be missing its reference card (or something similar). Nowhere in the manual (which is included) is any list of keyboard shortcuts.
The commands can be found easily enough elsewhere on the web but I was wondering if perhaps GOG could add a PDF to the list of downloads.

You might want to write to support about that. They are usually very good about rectifying situations such as these.

Support got back to me and confirmed M&M III doesn't include a keyboard command reference card. However, the keyboard commands are identical to those for the fourth and fifth games (World of Xeen) so you can look them up in the M&M 4&5 manual.