Here are the instructions from the guy who taught Brian this trick, Seth Rovner!
Have three cards laid out on the table. You memorize the middle card. Place the 3 cards face down. Have the spectator remember any card they like. With your back turned have them switch the two positions of the cards that they did NOT think of. Turn back around and tell them that they can mix up the cards on the table. You must follow the middle card which is the card that you remembered previously. You now look at the three cards, if the card you have been following is the same card you remembered it has to be that card. If that cards is a different card it has to be the other card that you did not memorize and visa versa.
Boom, you just did the Triple Threat Mind Reading Trick!!