Step 3. What's the odds of choosing the right door? 50%
Yes, but because this is a gameshow we can assume the host knows the winning door. Now he is not going to open a door with the treasure behind it; what would be the point in making a choice then? Its either lose or lose. So by switching you do infact move to a 2/3 chance of winning.

If it were in a non-gameshow scenario and the door to be revealed was picked from random it would still be a 50:50 chance.