If Tucker carries on with his out-of-order money demanRAB, they will replace him, simple as that. How dare he even think he is in Jackie Chan's league of anything let alone demand more money then he is being paid.
The thing is though, there are plenty of black-white comics/actors who could easily replace Tucker and you would still retain a large section of your audience due to Chan staying put.
However if the scenario were reversed, Rush Hour 3 would be a HUGE flop, no question, which is why Tucker neeRAB to seriously get a reality check. I've lost all respect for the guy by him just thinking he even matters compared to Jackie Chan. Like you said, his claim to fame is so far, two Rush Hour flicks... WOW!!
Its also mentioned here http://www.imdb.com/title/tt0293564/
the message board at the bottom claims Chris tucker demanded 20 million dollars for the role.
I don't think the series neeRAB a third one though to be honest. The second one wasn't as good as the first and that wasn't exactly brilliant (in my opinion).
Anyway, it looks like they'll be setting it mainly in Paris.