Jailbreaking has nothing to do with SHSH blobs. Your case is that you want to unlock. Even if you have iOS 6.1.2 blobs saved, you still can't unlock the device since there isn't an UltraSn0w fixer for anything above iOS 6.1.
And where can I find the 6.1.2.shsh blobs? I can't Jailbreack iPhone without it. I didn't know, that I must save it.
If you didn't save the SHSH blobs for iOS 6.1.2 when Apple was still signing it, there is no way to get those blobs after the fact.
OK. How can I use other carriers with my iPhone 3 GS?