My understanding is that you use Occam's razor on a list of possible answers. A third person isn't less likely unless there is some more likely choice. I don't know what that would be.
Edit: You're sort of glossing over that "nobody in the driver's seat" is unusual by itself. All explanations I can think of for how this happened are unlikely.
You don't need to have a "list" of explanations. Any explanation would either assume there were more than two people in the car (why stop at three?) or it would assume there were no more than two people in the car. Any explanation that assumed there were more than two people in the car would "multiply entities" more than any explanation that assumed there were only two people in the car, so the Razor would always cut out the former in favour of the latter.
>> Edit: You're sort of glossing over that "nobody in the driver's seat" is unusual by itself. All explanations I can think of for how this happened are unlikely.
That means you can't think of an explanation for it, not that there isn't one.
Again as I said in other comments, if you find yourself thinking "I can't think of a better explanation", that's a sign you need to sit down and think of a better explanation.
Edit: You're sort of glossing over that "nobody in the driver's seat" is unusual by itself. All explanations I can think of for how this happened are unlikely.