The glass part of the screen will fit perfecly, but you can't use the same LCD panel that's attached to it. You must remove all 6 screws from the glass frame that is holding the LCD panel in and replace it with the LCD screen from the 3GS. Do not try to use your 3G LCD in your 3GS because the connectors are different. I have taken apart an iPhone 3GS and tried installing a 3G screen and the 3G connectors didn't fit.
Good luck!