So essentially like this:

Original door has 33% chance of being good, compared to 66% chance of the other 2 doors

One of other two doors are opened by Monty, because he's a swell guy!

The two doors still have a 66% chance of being good, but since one of them is now opened, that 66% chance now only applies to the one other door

Ergo there's a 66% chance that if you switch doors, you'll pick the right door