What kind of 3G link do you get? I would assume that you get an IP link, but not a data link layer link, unless you booked an data center link, or something similar. So you won't be able to bridge anything here.
Next comes the ugly truth of mobile IPv4: you're behind a carrier grade NAT. While not a show stopper in itself, it may add a lot of problems. Then, IPv4 stacks behave differently when it comes to trying to load balance or bond multiple routes. While you have control over your side, you don't have control over your telco's exit routers, or the ones doing the carrier grade NAT.
Last chance: multiplexing individual TCP and UDP sessions. Again, you may control your outbound traffic, but your inbound traffic is controlled by your telco. So, only NATing possible on your side, but then I haven't seen this to the best of my knowledge.
Android does some multipath bonding via Wifi and data radio, but I don't know how this is done, unfortunately. It probably needs some tricks to work at all...
From ZX81 to Raspberry Pi, but wait ... where's the 7805 gone?