Correct me if I am wrong but couldn't you just bridge two connections such as a wired one of a the computer and the 3G modem? Then if you plugged your computer with the 3G modem into a router the computer would pass the Internet connection over into the wired network and from there you could use it normally.
First open up your network connections and hopefully your 3G modem will show up. Right click on it and select properties. There should be something about sharing. Share the Internet connection of the adapter.
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Then you can bridge the connections. Lets say that Network 4 is your wired network and Wireless Network Connection 2 is your 3G modem. Right click > Bridge.
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I don't have a 3G modem to test this with but it should work.