Bandwidth Aggregation with multiple 3G cards?

You could do this. Network Manager can do one connection per device just fine. But even with network-manager connected to multiple things, outbound traffic will only chose one gateway. You need to bridge these gateways into a virtual tunnel and use that as the gateway.

The problem is all the machinery is there (iptables, ipchains, iproute2, etc) is somewhat lower-level than network manager. You can try and follow a guide showing example configuration to split traffic over multiple gateways but network-manager might end up getting in the way.

It might not. It might work out just fine. You won't know until you've poked it ;)