If your 3GS is factory unlocked, try downloading iOS 5.1.1 from a third party website and check if iTunes allows you to manually upgrade to it. I doubt it will as (like I said before) Apple has stopped signing the firmware now and you will need SHSH blobs from Cydia. Considering you were never on iOS 5, you won't have any saved blobs.
You can upgrade to iOS 6.0.1
Just ensure your phone IS factory unlocked. You will rendered without network service otherwise and may have to rely on third party unlocking software, or purchase a factory unlock online.
No body can downgrade iOS officially
Apple has stopped signing iOS version lower than 6.0.1 
You can upgrade to iOS 6.0.1
Just ensure your phone IS factory unlocked. You will rendered without network service otherwise and may have to rely on third party unlocking software, or purchase a factory unlock online.
No body can downgrade iOS officially