Info-Mac

I downgraded it to 3.12 it's a 3rd gen it's a mc model yet how do I jailbreak it and cancel my warranty

I believe once you update to 3.1.3 it updates the baseband which will prevent you from jailbreaking.

Can I jailbreak 2nd gen 3.13

Oh yeh because while looking on the Internet I found one called blackra1n which jailbreaks it and it's a mc model

Yes you can jailbreak iPhone OS 3.1.3. I'm not going to list out the steps to do it here because it's widely available all over the interwebs, and since Apple is so unpredictable with who they "crack down" on information like that.

All mb 3g iPhones can be jailbroken with either snowbreeze or redsnow, baseband doesn't matter. 3.1.3 3gs, you might as well sell it cuz the jb won't be out for a while.

You fail spirit can jailbreak them untethered

spiritjb.com

yes you can, and you can find some tutorials through google:)