Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I used a mac and pwnagetool.

I backed up the phone, then downloaded the new "iPhone1,2_3.0_7A341_Restore.ispw" file. Ran pwnage, created a custom ispw and then restored the phone through itunes to that custom firmware. After all that, I just restored from a backup, and voila, jailbroken 3G running 3.0 software
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.