How to share 3G usb internet connection through wifi router?

[Fabio Pintos]

You should be able to use Internet Connection Sharing for this.
http://windows.microsoft.com/en-US/windo...on-Sharing

Other option is to create a "bridge" between the two networks.
http://windows.microsoft.com/en-US/windo...ork-bridge

I don't remember the difference between the two, but someone already answered that, so you can double check which works better in your case:
http://social.technet.microsoft.com/Foru...87aa8984cf

Note that in domain-joined machines ICS might be disabled by the administrator.