Martin, we do not believe that the universe popped out of nowhere. The thing is, we don't know. We just don't. It's possible (though difficult to comprehend) that the universe had no beginning and has always been here. It's possible that a god outside the boundaries of our universe created the universe. It is also possible that during a time when there was nothing and the laws of physics (namely thermodynamics) did not exist, that the universe did come from nothing (though I do agree that is far-fetched)
But bottom line is that we do not know, and so it is irresponsible for anyone to say either way. Until we know what happened prior to the big bang, we never will.
However, that does not mean that the idea of God creating the universe is as likely as any other. With the first and third ideas I proposed, they are within the boundaries of our universe; a place we know exists because we are in it right now. But with God, he supposedly lives outside the our universe. And since we have no evidence of anything existing outside our universe, the concept of god creating the universe is less valid.