The Fight Club director has only signed on to make the first book at the moment, which Daniel Craig has recently been cast in as the lead character, investigative journalist Mikael Blomkvist.
It’s understandable that Fincher is reluctant to sign on for all three films. That kind of commitment could keep him tied up for many years, and judging by the choice of diverse subjects he likes to work on (including the upcoming film on the Facebook phenomenon, The Social Network) he probably wants to keep his options open.