Q: how to use face time on 3 G network

Your wireless provider must support it over 3G. Otherwise you can't do it.

Facetime requires wi-fi

http://support.apple.com/kb/HT4319

At this time, it isn't possible. It will be available with iOS 6 which releases Sept 19th.

Yep, on Sept 19th iOS 6 will bring FaceTime to Cellular networks if your carrier supports it.

Here in the United States, it looks like AT&T is the only carrier that is actually charging you more to use it, you'd have to get one of their new mobile share plans to use FaceTime over Cellular.

Keep in mind it does use your data plan if you're on 3G so it could eat through some of that data limit pretty quickly.