Blackwood and Westhill are updated, both now have "free roam" ability out of the main track. Otherwise, the editor has been updated to be more efficient (in theory), and Rockingham has been released in S3 a couple years back, also free roaming.
Not much else after, if you are a fan of the Rock or wish to add another track to race on then by all means S3 is the way to go. You might also benefit from the odd race event at Rockingham and of course the added track to explore, but if I'm honest I'd stay out of it if you are contented with S2....
Until the new update the devs are working on gets released.