The iPhone 3GS is on 3.1.3 and has the old bootrom. F0recast says it can be jailbroken/unlocked without tether.

Should I use sn0wbreeze? and will I need the SHSH blobs when restoring with the custom firmware even though its already on 3.1.3?

Thanks

On 3.1.3 without SHSH blobs, your only choice is to Spirit jailbreak (hope you have a legit AT&T SIM!) and then Spirit2Pwn to pwn your LLB, then PwnageTool to create a custom 4.0, then shift-restore the custom 4.0.

Make sure you use firmware umbrella to get your SHSH for 4.0 regardless of whether or not you jailbreak. You'll want it for the future tools.

If you don't have a 3.1.2 or 3.1.3 SHSH now, you can't get one, so if this process fails and your phone bricks, you'll have to restore a FACTORY 4.0 onto your device, locking and jailing it until new tools are released, so, if that impacts your desire to jailbreak at present, at least you have all the information.