I know this thread is a little bit old, however I was wondering if anyone found any better solutions to the original poster's question. I have searched around the net with searches such as "broadband bonding," but I hoped to maybe find some other solutions around the forums.
It is possible to do this anywhere, best way though is to simply have a MikroTik device with multiple IP's in a data center with low latency to your 3G services.
This way each 3G link dials a connection (because we're assuming that the 3G IP addresses are dynamic so permanent tunnels are not an option) back to this data center housed device, which in turn completes the bonding process and forwards traffic out its own connection.
Yes, that method was what I used at the time to load balance across the 3 x 3G sticks we had in place.
If you know that one stick has more bandwidth than the others simple adjust the ratio of your PCC rules to match this.
For example if one stick has 14Mbps and the others only get 7Mbps each you would have 4 PCC outcomes, with 2 of these outcomes utilising the first stick and 1 each for the remaining 2 sticks.
It's not a perfect balance but on average it works the best this way.