Well dev team released "pwntool" ages ago but it only works on mac, if u have a mac or know anyone that has a mac they/you can make your own custom f/w which is pretty easy to do if u follow the steps. You can also find and download the custom 3.1 f/w people have made using "pwntools" by searching google, the reason why the dev team doesnt link to a d/l is because the f/w is copyrighted 2 apply so they would get into trouble if they did.
If u want gimme a pm and ill send u the links you need 2 j/b 3.1 BUT be warned if it f***s up your iphone its not my fault. also if u got the 3gs im not sure ur able 2 do it yet but i can still give u a few good sites u can check out with guides.