It’s done. iPhone OS 3.1 has been jailbroken for the iPhone 3GS now by the Dev Team. The catch: You need to have already Pwnd it at 3.0 or 3.0.1—you can’t jailbreak 3.1 directly, for now. [DevTeam]
https://gizmodo.com/iphone-os-3-1-jailbroken-and-unlocked-except-for-the-3-5360204