Not 100% on this as i have not tested this (lack of device) but this is how i would go about creating the Rule.
Device Definition would be Plug and Play Device Definition
Parameter Name would be BUS TYPE (USB)
This is the part that I am uncertain of
Parameter Name would be USB CLASS CODE (Wireless Controller)
Parameter Name would be DEVICE CLASS (Wireless Communication Devices)
Some devices i have found also register as Windows Portable Devices also listed under DEVICE CLASS.
Your Plug and Play Device Rule would then exclude the new Definition for these devices. You might want to see if you can tighten the rule using the serial number of the 3G device to ensure that only whats approved is used.