Well, if you only have two doors, Monty can't open a wrong door, so 3 is as scaled down as it gets.
Scaling down to 2 doors, your odds of winning are 1 in 2, or 50%, not 100

Scaling up to make it more understandable.

Let's suppose there are 100 doors and you pick one.
Chances of winning... not so good ... 1%

Now Monty opens 98 of the remaining doors and none of them are a winner.

Should you change your pick to the one remaining door not opened, or stay with your original pick?
It's pretty clear you should give up your pick and take the one door Monty didn't open.
Switching your pick gives you a 99% chance of winning.
The only way you lose is if you had picked the correct door on your first guess.

The chance of winning INCREASES as the number of doors increase.

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