Although I've never tried, the 2G glass will probably fit the 3G phone. However, it is completely insane to try and test that theory. It is EXTREMELY difficult to seperate the glass from the touch in both the 2G and 3G phones. I've heard that boiling the 2G display (the LCD, touch and glass are tightly glued together) will allow you to easily seperate the glass but destroy the LCD and touch. The 3G touch and glass are fused together but the LCD is a seperate piece.
If you want to try to fix the glass yourself, get the 3G touch and glass replacement part. The is your best (and really only viable) option.
By the way, although the 2G glass may fit the 3G phone, the sensors are in different places, so your 3G phone's proximity,light sensors won't work properly with a 2G glass.

To answer Tazeen, that is why it is so expensive to repair your 2G display. Even though it is only the glass that's broken, you need to replace the entire display.