The S3 license contains 20 cars and 9 tracks in total. In other words, all demo content + S1 content + S2 content + 2 additional tracks. S3 license also allows using and creating custom vehicle mods through an integrated mod system. In addition to more cars and tracks, buying a license enables up to 40 grid slots and 79 connections online, selecting reversed track configurations, building custom autocross layouts, uploading custom car skins without having to buy extra slots, and using all lfs.net website features.
The base price for S2 license is £36, however it can cost £27 or £18 depending on the user's country. You can see the prices for each country on the voucher selection page. If you already own an S1 or S2 license, upgrading to S3 will only cost the price difference.