Under Windows 3G mobile adapters normally appear as WAN adapter under NDISWANIP device of install their own virtual miniport (visible as LAN or WiFi connection). For the first case it is enough to filter NDSWANIP device, for the second you must track when new network connection becomes available and capture on this newly added network interface.
Another type of mobile broadband drivers (NDIS 6.2) was added by Windows 7. In the meantime these adapters can’t be intercepted by WinpkFilter NDIS IM driver. We are working on the adding NDIS LWF driver to WinpkFilter set of drivers to support this new stack.