I found the easiest way to downgrade from 3.1 b3 to 3.0.
WINDOWS

This method worked with Original Rogers 3G iPhone.

1. Have iTunes 8.2 Installed
2. Shift-Restore and select iPhone 3.0 IPSW file and let iTunes do it's thing. You will get an error (1015). Once you get the error you have 2 options:
3a. Use iRecovery to fsboot iphone (didn't work for me)
3b. Hold Power+Home button for 11secs and reset iPhone.
4. Restart iPhone by holding Power Button for 3secs
5. Your phone should be at the home screen (emergency call).
6. Now take the iPhone USB out and plug it back in. (this will allow you to see iPhone on iTunes again).
7. Thats it.

Optional Jailbreak 3.0
8. If all went well - now download Redsn0w 0.8 and run redsn0w and jailbreak 3.0.
If you have issues with redsn0w don't worry try it a few times it should work.
If you get stuck in "Waiting to Reboot" just reset the iPhone. (Hold Power + Home Button) for 5secs and then release the power button and keep holding Home Button for 15secs until you hear a sound on PC.
There you have it.
NO CUSTOM IPSW is required and you should have an operational iPhone 3G 3.0.