I'm not very good with networks, but wouldn't a network bridge be a possibility? If you open up the Network and Sharing centre, and on the left click on Advanced Adapter settings, does your 3G dongle show there? Can you Ctrl-Click on both the 3G dongle and ethernet > right click > bridge connections.
There's probably some critical flaw I've overlooked in this plan - as I say, I'm not a networking specialist, but it would be the first thing I would try if this were my own PC.
