if you want to downgrade to iOS 2.0 (im going to assume clean non jailbroken) you have to download pwnage tool 2.2.5 and the iOS 2.2.1 IPSW just open the app from the dmg (ignore the iTunes 7.7 message) with your iPhone 3G connected click on expert then follow the guide it provides to make a custom IPSW when you get to the is the phone already pwned hit no then follow the DFU guide it will then pwn your 3G and then restore the custom 2.2.1 IPSW in iTunes it should restore once thats done you can then restore to the stock iOS 2.0 IPSW and your 3G should be on 2.0
(this works fine on el cap 10.11.6 I have not tested 10.12 but it may work there too)