Depends. My back-of-the-envelope guess is that Ford would possibly survive, while GM could either reorganize and divest, or could disappear...and that Chrysler is the moist likely to go the way of Studebaker and De Soto. But go here for a 2005 study of U.S. Automotive Industry Employment Trends that should give you a pretty good idea. [It's a .pdf]
But it's not just the auto makers themselves, but the other industries that feed them *just in time* parts shipments, subcontract parts [everything from electrical systems to wheels] and steel and rubber suppliers and machinery and systems suppliers.
I don't think three million is at all impossible, just that it probably won't be 3 mil at once, but a cascading and worsening black hole of disappearing industries and jobs from which there may be no return.
I know three guys who are a lot sharper about these things than I am [one is an editor at WSJ who has a professional interest and the other is an analyst at CBOT] and I want to pick their brains a bit before I begin to yell that the sky is falling. But it wouldn't surprise me if it does, and the conditions are right for it...and other things.
I am skeptical about the number 3 million since most of those are in other than car/truck direct manufacture. Suppliers can supply whoever is building cars. They might also include Mike the gas station owner who can fuel and fix any piece of rolling junk no matter who built it.