I am trying to downgrade my iPod Touch 3rd generation from 4.2.1 down to 3.1.3, so I can Jailbreak. Everything I have tried has failed. And it keeps saying it is not compatible with my iPod. Please help.
Green poison doesnt work retard
You can use Greenpoison to jailbreak iOS 4.2.1. http://greenpois0n.com/For jailbreak status and tutorials on how to do the jailbreaks, you can check here: http://www.jailbreakmatrix.com/iPhone-iTouch-Jailbreak
you can't put the ipod in dfu once it's upgradet to 4.2.1
You can use Greenpoison to jailbreak iOS 4.2.1. http://greenpois0n.com/
For jailbreak status and tutorials on how to do the jailbreaks, you can check here: http://www.jailbreakmatrix.com/iPhone-iTouch-Jailbreak
Downgrading to iOS prior to 4.x requires the use of TinyUmbrella or something similar since those versions are no longer getting signed by the iTunes Server.
From your post I can't figure out where this "not compatible" occurs ~ whether it's from the Jailbreak Application or from iTunes. Either way I think you should take a look at Wikipedia about the possible options.