Jailbreak your iPhone 3GS on OS 3.0 and jailbreak it with redsn0w too......
Firstly download the required files from our iPhone and iPod touch Download Center.

Step One
Create a folder on your desktop called Pwnage

Download RedSn0w and place it in the Pwnage folder. Likewise, download the latest 3.0 firmware and place it in the same folder.

3.0.0 (3GS): iPhone2,1_3.0_7A341_Restore.ipsw

Extract the RedSn0w executable from the zip file to the Pwnage folder.


Step Two
Connect your iPhone to the computer and launch iTunes.
Select your iPhone from the list of devices on the left. Now hold down shift and click the Restore button. Restore is preferred as it won't create any wasted space on your iPhone.

Navigate to the Pwnage folder on your desktop and select the firmware ipsw. Click the Open button to continue.

Step Three
Once iTunes has finished updating your iPhone to the desired firmware you may launch redsn0w.exe from the Pwnage folder on your desktop.


Step Four
Once RedSn0w opens click the Browse button


Step Five
Select the firmware ipsw we placed in the Pwnage folder on our desktop then click Open


Step Six
Once the firmware has been verified click the Next button to continue.


Step Seven
Make sure Cydia is selected and click the Next button.


Step Eight
RedSn0w will now instruct you on how to put your iPhone into recovery mode. Disconnect you iPhone 3GS from the computer then power it off. Then hold down the Home button while reconnecting the device to the computer. Don't let go of the button until RedSn0w detects the iPhone and begins installation.


Step Ten
Your iPhone will now reboot


Step Eleven
RedSn0w will then begin uploading the new RAM Disk and kernel.


Step Twelve
Once this is complete you will be notified that RedSn0w is done. Click the Finish button. When your iPhone finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard.


Now you are done................