You will most likely need to jailbreak you 3G phone. And then install a tethering app, such as MyWi, or PDANet.
From what I understand, does MyWi only support Adhoc wifi networks, which the iPad cannot join. PDANet also only creates Adhoc networks.
Anyway, if you want to use wifi tethering, you will need to jailbreak both devices and install either of the above mentioned apps on both devices. Then simply create the same wireless network on both (same Name and password). It is slightly anoying to tether both, as it drains the battery quickly.
The only alternative is to just tether the phone and then tether via Bluetooth through one of the apps.