It does not work with an iPhone 3GS.
FaceTime only works when the device is connected to a WiFi signal, and only the iPhone 4 allows you to share your iPhone's 3G signal either through Bluetooth or WiFi (also USB for a Laptop), when using an iPhone 3GS, your only option for tethering is Bluetooth or USB.
I tried this like two weeks ago and I got a Pop Up asking me to go to Settings and connect to a WiFi signal.
I'm not sure about Skype, but Tango worked just fine from an iPod touch tethered to an iPhone 3GS.