i need to know that if i update ma iphone 3g on 3.1 after that i can unlock it with custom firmware
if yes then plz give me the custom firmware link
and tell me one thing that what is the modem firmware if i upgrade ma iphone 3g in to 3.0.1

3.0 and 3.0.1 have the same baseband 04.26.08

Upgrading to 3.1.x and keeping this baseband requires a custom ipsw made with pwnagetool. How to Jailbreak Your iPhone 3G on OS 3.1.2 Using PwnageTool (Mac)

It is against the rules of the forum to link directly to custom ipsw made with pwnagetool, you either need to make your own or use google to find one pre-made. You must also realise that not all of the custom ipsw files floating around are created equal. Some patch other things that you might not require and might impact the stability of your iPhone.

Far worse, is that some custom ipsw files do not disable the baseband update part of the 3.1.x ipsw, leaving you with the 05.11.07 which cannot currently be unlocked.

I would very much recommend that you make the ipsw yourself and if you don't own a mac to borrow a friends mac for the 20 minutes that it takes to make this ipsw and load it on your iPhone.