You can unlock the i-phone 3GS simply using the remote unlock service..it's simple...But i suggest the simple way to unlock the,instead of using the hardware and firmware,you can better use the remote unlock service to unlock,You can get the remote unlock service at here
Classicunlocking.com and unlock it.
Apple iPhone 3G S Unlocking Instructions
1. Wait for us to send you an email saying your phone has been activated or remotely unlocked.
2. Install the latest version of iTunes.
3. Make sure your Apple iPhone 3G S phone software is updated to the latest version using the iTunes software.
4. Connect your Apple iPhone 3G S to iTunes with a
NOT ACCEPTED SIM card.
5. Wait until iTunes detects your Apple iPhone 3G S.
6. Disconnect your Apple iPhone 3G S and reconnect it after 10 seconds.
7. Your Apple iPhone 3G S is now unlocked.
To jailbreak the I-Phone 3g s you can reach here
Jailbreakiphone101.com