I just finished running this ccampaign for my players. It rocked, but required some tweaking.
You'll find a 3e conversion in the conversion library. It needs some hard editing and a careful look through, but it's workable. The third book is probably the weakest conversion, but I was busy when I wrote it.
Spoilers below:
From a player perspective, Book I opens up wiith a delivery job to the sleepy little county of Haranshire. There they become involved in investigating the disappearance of a wizard's apprentice. One thing leads to another and they discover bandits are behind the disappearance of her and many other spellcasters. They in turn find out that the bandits are being paid well to turn those spellcasters over to a tribe of orcs living beneath the ground.
A visit to the caverns of the orcs reveal both the sinister hand of Mind Flayers behind the orcs and some svirfneblin that might be new allies.
That ends Book I
Book II opens with the party having to prove themselves to their new allies by investigating the Night Below. Their first task to earn the trust is to wipe out a lair of trolls. 40 trolls spread out over several hundred feet of cavern space. Depending on what the party did prior to this, they should be around 6th level.
Presuming they are successful they will have begun to earn the trust of the svirfneblin and will be told that the slaves are being taken to the City of the Glass Pool, a kuo-toan citadel. The party is given a map and instructed to make their way there for the answers they seek. They are also instructed to slay all the derro they find - apparently there's a lot of them.
After many encounters, good and bad, the party finally reaches the City of the Glass Pool. There they will most likely have to slaughter a large number of Kuo Toa and the Mind Flayers that lead them in order to reach the passageway that leads further down. For the City is but a way stop on the slave trade of spellcasters.
Book III opens with the party taking their first steps into even deeper tunnels, eventually finding their way to the Sunless Sea, a massive body of water below the earth. Exploration will reveal enemies as well as allies of uncertain intent. Eventually the party will make their way to the lair of the architects behind all the kidnappings, the City of Shaboath, full of aboleth and their minions. They will also have learned the aboleth's grand scheme; the total domination of the underdark.
The aboleth have constructed the Towers of Domination, a massive artifact that will extend their natural enslavement ability to every place beneath the earth. The towers are not yet complete, but they will be soon.
The climax of the book III is when the party assaults the City of Shaboath to achieve certain tactical goals and to ultimately assault the towers of domination. If all goes well, the party will meet the magical genius behind the towers as well as the savant leader of the aboleth.
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Having run it through to completion I'd make some different emphasis' through the module.
1. Determine where the derro minions come from. There are hundreds, but they have no city of their own; yet the svirfneblin hate them with a passion.
2. The magical architect of the city is a derro wizard - yet there is no mention of him at all until the party meets him.
3. Under 2nd Edition, the party was expected to be around 14th level when they completed the module. In real play, my party was 14th by the time they hit the end of Book II. Some were 20th by the time they finished. That...changes things. Plan accordingly.
4. Plan on some additional out of dungeon encounters for variety. Most complaints I've heard is that Book II is a dungeon slog....way too long. If it's broken up with some surface adventures though it won't be too bad.
I'm sure there are other things, but those are off the top of my head.
I've got a story hour (link in sig), which currently goes up through the middle of Book II. I haven't had any new additions to it for a couple months, but there will be additions.