This game picks up where Zork 2 left off (minus your inventory- I really could have used that magic wand!). You're stumbling down an endless stair to a cavern where you find your old friend, the brass lantern.
This game departs a bit from the first 2 games, in that the object is not to find all the treasures and drop them in a case. You're still looking for all the treasures, but they aren't apparant as such, and the game is looking for certain behaviors from you.
One complaint on this game is that one of the puzzles (the most important one, you might argue) is timed, so in order to gain the permanent light source, and one of the treasures, you need to do the puzzle RIGHT AWAY, otherwise you render the game unwinnable. And in Zork 3, it is easy to make the game unwinnable and not realize it.
It was possible in Zork 1 and 2 also, though it was much more apprarant- if you died at the volcano and you left some treasures in the balloon- they were unreachable. In Zork 3, you need to decide at one point whether to go for a staff or treasure, how to respond to a mysterious viking ship, choose between to solutions to a shifting wall puzzle, decide what items to try to steal during a time travel puzzle, decide whether to kill someone attacking you or not (and the choice is not obvious),decide WHEN to do a puzzle involving teleportation- and the wrong selection on any of them makes the game unwinnable, and you never realize it as such unless you go back and do things the RIGHT way.
Now, I don't know that this is UNFAIR, because I like difficulty, I would only wish I knew what I was supposed to do before I screwed myself up. If you do what many people might and explore the entire world right away, you've already lost too much time.
That being said, some of the puzzles are freaking BRILLIANT! A puzzle where you need to slide a mirror is difficult to visualize, but very smart. The shifting room puzzle gave me that real "AHA" moment as well. The time travel puzzle makes sense when you think about it, it's just not exactly clear how time travel relates to the time machine itself. If you're a fan of Zork I and II then you shouldn't be really surprised by the solution of the mysterious ship puzzle, and you should relish the chance of being able to walk past some grues in the dark. (A feat you will repeat in Spellbreaker, and possibly in Sorcerer).
The game does tie up the trilogy nicely, provides a good ending point, and gives you the challenge you deserve, without bogging you down in inventory management (very much) or much of a light puzzle (if you run out of light you either missed the first puzzle or did something stupid, like entering a lake with a torch).
If you like Zork I and II you will like this as well, just be ready for a bit more serious a tone and more difficult puzzles.