radarmark,
re: "If there is a car, and if the only choices left are curtains 2 or 3..."
After the host opens curtain #3, the only choices left are curtains #1 and #2. Curtain #3 is out of the picture. There is a 1/3rd chance that it is curtain #1 and a 2/3rds chance that is curtain #2.