Snowbreeze is one of the most used jailbreaking tool used to jailbreak your iphone easily using Cydia. . Sn0wbreeze released it latest version sometime back  sn0wbreeze 3.1.3 which can be used to  jailbreak iPhone 3.1.3 2G, 3G and 3GSm3G[S] devices with old boot ROM. Snowbreeze is used mostly because Snowbreeze 3.1.3 will preserve the Iphone Baseband during jailbreaking which will be secure one with out any problem..and hence unlock your iphone using Ultrasnow 0.92 app. which can be found in Cydia..

Here is how it can be unlocked your iPhone 3G/3GS 3.1.3 with Ultrasn0w 0.92

* Jailbreak your iPhone using Sn0wbreeze 3.1.3 for iPhone 3G & 3GS 3.1.3 and Redsn0w for iPhone 0.9.3
* Once you jailbreak it Open Cydia 9Internet connection is required)
* Once you did it Now type http://repo666.ultrasn0w.com source URL and click on Add sources
* Search for ultrasn0w 0.92 and install it to automatically to unlock your iphone
*Once you do it a  Reboot is required and  your iPhone is unlocked and you can enjoy  unlocked iPhone 3G/3GS 3.1.3