If you mean unlocked, then yes. You'll need to create a custom IPSW to preserve your baseband using RedSn0w, restore your 3GS with it, then jailbreak it. After jailbreaking, you'll need to install UltraSn0w again, and if you're on an iOS 6 jailbreak, you'll also need to install the UltraSn0w Fixer for iOS 6