Think of it this way…
You pick one of the three doors.
The odds that you picked the right one are 1 in 3.
The host opens a door and shows you a goat.
At this point, before offering you a choice, would you agree that the odds that you picked the right door are still 1 in 3?
By the host opening the door, we know that the odds of there being a car behind that door have collapsed to 0, while the odds that you picked the correct door to start with is still 1 in 3, you haven’t been asked if you want to change yet.
As the set of all odds has to add up to 1, the other door then must have odds of 2 in 3 that the car is behind it.
Still no choice been allowed here.
At this point you are asked if you want to change. Can you explain to me why the odds would then change to 1 in 2 at this point?