While clearly the first two were beyond excellence, the third one isn't bad. I thought it would suck, but then I played it, like a year ago, and it's actually really fun, like an expansion with better graphics. Really, the only major thing missing is the quality NCP banter.
Same thing that killed Dues Ex 2: shitty cramped levels. You would literally knock out one dude in a corridor, then pass into another load zone where there would be one more guard who couldn't discover any bodies because they were in a different load zone.
Thief 3 and Deus Ex 2 used the same engine! The result for both was teeny tiny levels with huge load zones inbetween. That engine was the fucking grim reaper of once great franchises.