The install of my ATI driver in FC2 (kernel 2.6.7) went perfectly as described in the FedoraFAQ:
I don't know if it will work for FC3. But that's where i would start. This forum's member Micha wrote the FC2 patch, and I noticed that he replied to another post in the FC3 testing section from a member trying to do what you are. He could probably walk you through any problems you encounter.
The only difficulty I had with the procedure was that I inadvertently stopped the kernel-sourcecode install after downloading the binaries but before they were compiled (The yum script gives a message at the end of the download that says something like "Test transaction completed. Success!" which I interpreted to mean "I'm done." It wasn't
) Repeating the source install and waiting until a command prompt was displayed fixed it.
Best of luck--let me know how it works.