If you want jailbreak iPhone 3GS with firmware 3.0 using Redsn0w 0.8. Now you can do it by using simple steps guide but you firstly must have updated or restored to the original iPhone OS Firmware 3.0 on your iPhone 3GS using iTunes 8.2 before make any move, after ward you can follow all the steps from the following link:
http://www.redmondpie.com/how-to-jailbreak-iphone-3gs-with-redsn0w/
Good luck!
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