Here is the 3.1.3 downgrade for the 3GS to 3.1.2

OK.. I found it. I did it and it works. It's a 2 part video. To many instructions to type. It's for real. Hope the admin's don't take out the links. Just trying to help. Leave comments if it works or if it doesn't. Also, once your done. Download Fuzzy Band on cydia to downgrade your basebadn so you can unlock the phone.

http://www.youtube.com/watch?v=9t9kLJhbpWU
http://www.youtube.com/watch?v=OZ8w6qR8d2M

 

As I read on, If you don't have 5.8 bootloader or lower. YOUR BEAT.. No unlocking at this time...

 

I ran into another issue. I have a few Iphones to downgrade. It will only let me do one. If anyone can figure out why and or how to by pass that. PLEASE post !!