Thief 3 is...not good. Half the game is spent in a generic city hub where you're constantly chased by guards who are always on high alert because they blame you for dead bodies from NPC in-fighting. This becomes particularly stupid (and hilarious) in the finale, where the guards are still focused on you while there's a Lovecraftian monstrosity roaming the streets. Oh, and the guards remember your position when you pass through a loading screen into another district of the city so if you return there the guards will still be there and instantly kill you after the loading screen. Brilliant design.
The other half of the game consists of a series of serviceable stealth missions that aren't remotely as good as the missions from the first 2 Thief games. And no, the mod that removes the loading screens doesn't remedy the downgraded level design. There's no swimming, and the climbing gloves are a poor substitute for the rope arrow since the game is very restrictive about what walls you can climb on. Sound seems to matter way less than it does in the first 2 games: often I found I could run up to an enemy and knock them out without my footsteps alerting them. Mission objectives are less interesting too, with a generic 'find 3 special loot items' objective being reused for every mission. There's a variety of non-human enemies but it doesn't matter because almost all are interchangeable with human enemies, to the point that even zombies can sprint. And I never liked that Garrett's sword was replaced with a shitty dagger.
Story is another major downgrade from the first 2 games. After the Pagans and the Builders, we were supposed to get an exploration of the Keepers, but they completely botched it and ruined the mystique the Keepers had in the first 2 games. I couldn't even tell you what the story of Thief 3 is about because it's so dull and unremarkable. Equally bad was the decision to have pre-rendered cutscenes that look worse than the in-game graphics and have some of them occur in the middle of gameplay. They should've stuck to the storytelling style of the first 2 games.
Even if the missions were amazing (they aren't), I still wouldn't replay it because of the slog that is the city hub.