As all of you know FaceTime was introduced in iPhone 4 and it is also included to iPod Touch 4G. But folks at iPhoneIslam have released FaceIt-3GS application which enables FaceTime video call on iPhone 3GS as your device should be jailbroken.
How To Enable FaceTime On iPhone 3GS
1- Launch Cydia on your jailbroken iPhone 3GS on iOS 4.1
2- Add the following repo: http://apps.iphoneislam.com
3- Search for Faceit-3GS, install and reboot your iPhone 3GS
4- Then go to Settings > Phone and simply turn ON FaceTime
That is all now you have enabled FaceTime on iPhone 3GS with Faceit 3GS.