I don't think you can... except for this rather perverse way, which involves a Mac:
1. connect your Mac to an iPhone which permits tethering, or establish a Personal Hotspot to your 3G network.
2. start the Facetime app on your Mac and make a Facetime call.
Why anyone would want to do this is another question, because you still cannot make calls to other iPhones through 3G (it has to be to another Mac or to another iOS device through Wi-Fi), and to lug around a Mac just to make Facetime calls is probably something that no-one would want to do.
Facetime requires an Internet connection and an Apple ID; what type of Internet connection shouldn't matter, but my iPhone 4 won't allow Facetime calls through 3G, no matter how hard I try. Apple's on-line literature also says:
"Requires a FaceTime-enabled Mac with an Internet connection or a FaceTime-enabled iOS device with a Wi-Fi connection. Not available in all areas." (http://www.apple.com/iphone/built-in-apps/).
so I don't like anyone's chances.