King's Quest III introduces an automapping system to the genre: a magic map, found in the game, can be used to teleport to most of the explored locations.
King's Quest III is the only AGI game (i.e. a game using Sierra's AGI, Adventure Game Interpreter) I know of in which turning the sound off causes an effect besides just silencing the game: In the wizard's laboratory, when you prepare the spells listed in the manual, some background music normally plays while you work, but if you turn the sound off, the game instead subtitles the experience by displaying a message reading "A mysterious music fills the laboratory!" when you start, and if you mess up on making the spell, another message pops up saying "The mysterious music stops. What could this mean?" It's a small thing, but notable since this kind of subtitling wasn't common in Sierra's graphic adventures.
This is the first game of the series in which the characters have pink(ish) skin. Although in the final scene of the game, when you return with Rosella to the King and Queen, King Graham, still has yellow skin, as he did in the first two games. (Maybe he suffers from jaundice, or was born in Springfield?)
While not being a formal copy protection scheme, you needed the manual to complete this game. It contained spells that you eventually used to advance in the quest. However, Compute's "Official book of King's Quest" did list all of them and one could purchase a copy for half the price of the game if the manual was "lost".
In the years after KQ3 was published, the idea of using the manual as a copy protection technique became a de facto with almost every games until the CD-ROM replaced diskettes as the distribution media.
If you had CGA and the wizard used a powerful spell, the entire screen shook. (This was an intentional special effect.) This was accomplished by tweaking the CGA registers to scroll the screen left and right rapidly.