For iPhone 3GS Unlock with Untrasn0w here and Jailbreak here.
If you want to build custom firmware files with more flexibility it is suggested that you use ‘PwnageTool’ on Mac OS X.
Before you start. Checklist:
1. GOLDEN RULE: If you are using a 3G iPhone with yellowsn0w and rely on yellowsn0w to obtain cellular service, then you should NOT use redsn0w right now. Ultrasn0w (the 3G carrier unlock) is not included with this release and therefore your baseband will be locked and unable to use an operator other than the official one it was bought for. UltraSn0w will be released via APT (cydia and icy) soon. If you have an original iPhone (1st generation) then 3.0 carrier unlock works with this redsn0w release.
2. Yellowsn0w in its current form will NOT work with the baseband version that is present in the 3.0 update, you will need Ultrasn0w, which will be released sometime soon, Ultrasn0w will work with all iPhone 3G models (but not 3GS), even ones that were previously unlockable, Ultrasn0w (when available) will be released via APT (this means you will be able to get it via Cydia or Icy).
Steps to Jailbreak, Unlock, Activate:
1. To use redsn0w v0.72 simply upgrade the device in iTunes to firmware version 3.0 and run redsn0w to activate and jailbreak the device (and if you are using an original iPhone 2G, it will unlock it too!) Official Links:
i. Make sure your iPhone is both turned off and connected to the computer. Hold down the Hold (top left corner) button until the ‘Slide to power off’ message appears, and then slide the button to turn off the device
ii. Hold down the Power button (top left corner) button for 3 seconds…
iii. WITHOUT releasing the POWER button, also hold down the Home (bottom center) button for 10 seconds…
iv. WITHOUT releasing the HOME button, release the Power button. Keep holding the Home button for 30 seconds…
DFU MODE SUCCESSFUL
Now Redsn0w should successfully terminate and iPhone will continue the process and Reboot. 🙂
Happy Jailbreaking and Unlocking!