FaceTime was introduce in iPhone 4 and iPod Touch 4G, now iPhoneIslam guys able to enable FaceTime option on iPhone 3GS as well. Your iPhone 3GS must be jailbroken to enable FaceTime video calling. See how to jailbreak iPhone 3GS or continue to enable FaceTime.
Enable FaceTime on iPhone 3GS
Run Cydia on your iPhone 3GS.
Tap Manage tab > Sources > Edit > Add buttons.
Type this repo URL
http://apps.iphoneislam.com and tap “Add Source”.
Faceit-3GS, install it and reboot your iPhone 3GS.
Now tap Settings icon > Phone and make sure FaceTime is ON.
All done, you have successfully enabled FaceTime on iPhone 3GS.