I'm pretty sceptical about the Riddler appearing in Batman 3 to be honest.
Everybody's expecting Nolan to re-imagine the Riddler much like he did with the Joker.
But I'm of the opinion that all previous screen Jokers never really fufilled the potential of the character that existed in the comics. Long time fans of the Batman comics, although appreciative of Romero, Nicholson and Hamill, always believed the Joker could be taken much further on screen (and what we eventually got from Nolan and Ledger far exceeded our expectations)
What was great about TDK is that Nolan plundered all the great Joker stories from the past (Batman #1, The Killing Joke, Arkham Asylum et al) when creating his Joker, and of course adding his own twist, to create arguably the greatest screen villain of the last twenty years.
My point is, those iconic Joker stories were there for Nolan to pick and choose elements from - but what iconic Riddler stories are there from the comics? I can't think of any. Are there any?
I'm just of the opinion that the Riddler is not a good character. And as a reader of the comics I always got the impression that Batman writers and artists never really liked him either. He's not an A-list villain like the Joker, Dent or Catwoman. The Riddler's always been a B-lister as far as I'm concerned. I'd rank him below the likes of R'as Al Ghul, the Scarecrow and the Ventilloquist as well!
That said, if Nolan were to use him in some capacity I'm sure he'd do something compelling. But he'd probably have to completely rebuild the character from the ground up.