Is it possible to disable the automatic download of updates when the internet connection is through a 3G dongle?

Everytime a network interface goes up, scripts in /etc/network/if-up.d/ get executed. You need to find out wich interface does your 3G connection binds.
This is a snipped for samba script, that comes in Ubuntu 12.04

#!/bin/sh
# Try to bring nmbd up when an interface comes up, if smbd is already running.

# Don't bother to do anything for lo.
if [ "$IFACE" = lo ]; then
        exit 0
fi

You can disable automatic update of the package list and unattended upgrade thorugh this file (this is for Ubuntu 12.04, your files might have different names), you can search for this pair of keys on directory /etc/apt/apt.conf.d

$ cd /etc/apt/apt.conf.d/
$ grep Update-Package-Lists *
10periodic:APT::Periodic::Update-Package-Lists "0";
20auto-upgrades:APT::Periodic::Update-Package-Lists "0";

Same goes to this key

Unattended-Upgrade "0";

You can build the script that copies the "non-auto-update" version of the files, when IFACE is your 3G connection. And in any other situation, copy the normal files that you have now. Here is an example that does something similar: How do I make the script to run automatically when tun0 interface up/down events?