Actually you can do this with Fring. Install it on the iPhone 4 and the 3GS, but make sure the 3GS is on iOS4 or 4.0.1. You get two-way video calling fring to fring but the feed will be coming from the back of your phone. Kind of pointless unless you are standing in front of a mirror.
Yeah bro, but first you have to do something called "Jail Break" where you submerge the phone completely in water for 4 and a half minutes. This allows you to connect to the mainframe where Saurik will then spoof your phone as an iPhone 4 toward Apple's servers. Live long and prosper, homie!
Jokes aside, FaceTime does not exist on the 3Gs through any hack that I know of. There are only the other workarounds listed in the previous posts.