Jailbreak your phone
There is plenty of info online about how to do this, so google it. As far as I know, the iPhone 3G and 3GS can be jailbroken on the most up to date firmware.
Any jailbreak method should leave you with Cydia. So in Cydia, search for MyWi. This application enables the phone's 3G service to be shared over WiFi. I don't condone pirating, so seriously, BUY this, don't steal it via those sinful iPhone repos that are out there that hosting cracked apps. Make sure you get the No Rock version with Cydia. If you are using Rock as your jailbreak package manager, then get the appropriate download.
Name your network, set a password. Then hit the on switch to enable to WiFi tethering. You'll know its working when you see the blue bar telling you upload and download figures.
Go to your iPad Settings->WiFi. Find the network that MyWi has created. Join it and enter the password.
But be careful of your network operator charging you for extra data, since tethering is technically not part of your "unlimited" service, especially here in the US with AT&T