In case u paid 12$ for S1 you can now enter to S3 with 24$. Anyway it is not lost of money as every license level cost same amount of money depending which country you are living. For getting only S3 content on top of S1 is not possible, and in S3 there is still only one new track environment offered. So best deal still is to go until S2 if you are not an absolute fan of Rockingham track.
Awesome, thanks Flotch and jkat - where can I go about doing that? How do I ensure I get S3 for 24$ when I place an order? I saw on the site they specify s2 for 24$ and I don't want any confusion, I do want to support the game further by purchasing s3 but I don't want to end up wasting money unnecessarily in lieu of that goal