You'll want to switch doors to have a greater chance of winning the car. You have a 2/3 chance of winning the car after Monty reveals the goat behind door 3. Think of the different scenarios presented.
Scenario 1: You pick the car. Goat 1 is revealed. You switch to goat 2.
Scenario 2: You pick goat 1. Goat 2 is revealed. You switch to the car.
Scenario 3: You pick goat 2. Goat 1 is revealed. You switch to the car.
You only lose if Scenario 1 occurs, which is 1/3 of the possible scenarios. The other two times, you switch to the car. Since you pick the door before the goat is revealed, and since the contents of your door do not change, the probability "concentrates" to the door you switch to. This is because revealing one of the goats doesn't change the fact that you picked a door when there were three to choose from.
Scenario 1: You pick the car. Goat 1 is revealed. You switch to goat 2.
Scenario 2: You pick goat 1. Goat 2 is revealed. You switch to the car.
Scenario 3: You pick goat 2. Goat 1 is revealed. You switch to the car.
You only lose if Scenario 1 occurs, which is 1/3 of the possible scenarios. The other two times, you switch to the car. Since you pick the door before the goat is revealed, and since the contents of your door do not change, the probability "concentrates" to the door you switch to. This is because revealing one of the goats doesn't change the fact that you picked a door when there were three to choose from.