How to Jailbreak and Unlock iPhone 3G on iOS 4.3.5?

Hello Dani, sad to say, but you will not be able to upgrade the firmware of your iPhone 3G to iOS 4.3.5 due to the fact that this firmware does not support your hardware. You can only restore your iPhone 3G to iOS 4.2.1 IPSW (baseband 05.14.02). Regarding Ultrasn0w carrier unlock, the only option available to free your iPhone from its contracted mobile phone carrier is by updating its baseband to iPad’s iOS 3.2.2 IPSW 6.15.00 baseband. However, you should be note that updating iPhone 3G’s baseband to iPad’s 6.15.00 might lose you the GPS functionality of your iPhone.

Anyway, you can check this blog post to know more and download the stock iOS 4.2.1 IPSW firmware file for your iPhone 3G. After restoring iOS 4.2.1 on iPhone 3G, you can follow this step by step guide on how to update the baseband version of your iPhone 3G to iPad’s 6.15.00 baseband using Redsn0w. You can refer on this tutorial on how update iPhone 3G baseband (05.14.02 to 06.15.00 | 05.16.01 to 06.15.00 | 05.16.02 to 06.15.00). Then refer on this guide on how to unlock iPhone 3G using Ultrasn0w once you have completely jailbroken your device.