This is a guide on how to jailbreak the iPhone 3GS with 3.0 firmware using redsn0w 0.8 for windows. The process is identical to all the other previous redsn0w guides. This will just jailbreak your iPhone. It will not unlock it. To unlock the iPhone 3GS use ultrasn0w (info here). This guide is for 3.0 only. NOT 3.1.
Your iPhone 3GS should already be on firmware 3.0 before you begin.
First download redsn0w 0.8 (windows)
Download 3.0 iPhone 3GS firmware .ipsw
Extract and then run redsn0w. Click browse.
Find your iPhone 3GS 3.0 firmware file. Select it and click Open.
redsn0w says your IPSW has been successfully identified. Click next.
redsnow starts doing its thing.
Select your options. Click next.
Follow the directions to get your iPhone into restore mode.
redsn0w starts uploading everything to your iPhone.
You will see the done screen. You can click finish at this time. On your iPhone you'll see some text and the pwnage icon for awhile as your iPhone is being jailbroken. Do not mess with it during this time. When it is finished your iPhone will be on the lock screen fully jailbroken