Here is how I understand this. Initially, the probabilities are as follows:

1. Car behind Door 1: 1/3.

2. Car behind Door 2 OR Door 3: 2/3.

If Door 2 is opened, and has a goat, the probability of the Door 2 / Door 3 combination STILL remains as 2/3. And as Door 2 has a goat, for sure, then the probability of there being a car behind Door 3 is now 2/3, while that for Door 1 remains at 1/3..