CS3 is too old for upgrade pricing. I believe only CS5 and CS5.5 have upgrade pricing.
If you really want CS6, you can get pricing (full price) here:
This is the US pricing. Choose your country at the bottom of the page, if you're in a different country.
Keep in mind that CS6 is 3-1/2 years old, and will never be updated. It's likely that it will not be supported in future operating system versions.
Would you try is to update from an historic ancient version to an old version. Both are at EOL, end of life. I would strongly recommend not to throw money to an old version. From CS3 is no upgrade path available anymore.