The logic is sound If you think of it like in groups. You pick your card and then the other 2 are put into one group. Your 1 card has 1/3 chance so the other 2 as a group have a 2/3 chance. If one of the cards in the group is shown not the be the card you are looking for the group still has a 2/3 chance of being the card you are looking for.