purplera1n for mac was released, which helps to jailbreak the iPhone 3.0 OS in the iPhone 3GS and this tool was released for windows earlier, works perfectly fine as tested on both the OS.

Here is a short tutorial on how to Jailbreak the iPhone 3GS using purplera1n on your Mac OS X –

• Download Purplera1n for mac (Download Link)
• After downloading, open the application. It comes as a zip file so first extract it.
• You should be having iTunes 8.2, if not, upgrade to 8.2 version without which the Jailbreak is not possible.
• Connect the iPhone to your mac
• Hit on “make it ra1n” in the purplera1n app

The iPhone enters the recovery mode, so you get notified that purplera1n is waiting for the iPhone

Then you see a picture in your iPhone with a boy, which is the Jailbreak image from geohot team like the pineapple when you use Pwnagetool or QuickPWN.
Now after its done, you get notified, and you will see a new app called Freeze with a blank icon in the iPhone. Open that.

On Opening the Freeze app, you can install Cydia as shown below. Cydia is an app that helps in bringing in the 3rd party applications to your iPhone.

Once Cydia is installed, it confirms that you have successfully jailbroken your iPhone 3GS.

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Here is a tutorial on how to unlock iPhone 3G 3.0 firmware with ultrasn0w(unlock) and redsn0w(jailbreak). But that is for 3G, and the above tutorial is for the new iPhone 3GS phones.