My friend has a fifth option available for his soldier class in multiplayer called "Battlefield 3". It looks similar to the human male model, but features the characteristic glowworm look as well as the Carnage ability in place of Concussive Shot.
How do I unlock the "glowworm soldier"?
You need to own Battlefield Three on Origin. For the consoles, at least, you only need a save file.
See here.
From what I've been hearing, the Battlefield 3 Soldier comes from the Starter Pack for those eligible. I'm not sure if there's a way to unlock him if you acquire Battlefield 3 after starting the Mass Effect 3 multiplayer.
