Here are the steps that has to be followed in-order to enable FaceTime video calling on your iPhone 3GS.
The first step requires you to launch Cydia from the SpringBoard and press to select the Manage tab at the bottom of the screen.
Now press the large Sources button.
Again press the Edit button i.e at the top right of the screen.
Select the Add button that appears at the top left of the screen.
Input http://apps.iphoneislam.com then press the Add Source button.
Press the large Return to Cydia button.
Tap the Done button to continue
Press to select iPhoneIslam from the list of sources.
Press to select FaceIt-3GS from the list of packages
Press the Install button at the top right of the screen and tap confirm button to begin installation
Tap the large Reboot Device button. When your iPhone reboots, tap the Settings button on your SpringBoard.
Press to select Phone from the Settings Menu.
Move the FaceTime switch to the ON position.
FaceTime is now enabled and you can begin video calling