You wouldn’t be able to monitor the 3G flow via the mac running tcpdump. I don’t know if there’s one for iOS (and thanks to Apple and Cisco using “ios” to refer to two completely different things, searching on “ios tcpdump” is useless).
I wonder if you turned off Cell data (settings/general/network) if the traffic would route over wifi or if it would just stop the activity until you turned cell data on.