This is a distraction allowing you the time to prepare to double lift the next two cards whilst they glimpse their card. Next, effect the double lift, and using it, show them the Queen of hearts with the ace of course hidden. Place the cards back on top of the pack, then remove the ace, naturally without showing them the face and make a jokey pretense of switching the cards a couple of times then say something along the lines of: ” Are you sure which Queen is which? OK I’ll help you sort it out! “

However, on this occasion, effect the switch very slowly, and really do switch the cards. (try to make sure that they don’t look at the ace in their hands) then, very quickly and as discretely as possible, take the Queen of diamonds which you are holding and place it on top of the deck. Next, “steal” the card off the bottom of the deck (Ace of clubs) and hand it to the spectator, asking them if they can now tell you which is the Queen of hearts. David Blaine effects this card steal incredibly smoothly and with terrific misdirection at 1′-12 in the above video showing his superbly honed skills as a card magician. When they’ve had a guess, ask them to turn over their cards and they are sure to be stunned to discover that they are in fact now holding 2 aces!!

Here below is another video showing a slight (possibly easier) variation and  on the same 3 Card Monte card trick. There is also an excellent explanation following the performance of the trick which should help you thoroughly learn it. The main difference with the David Blaine performance is that unlike Blaine, the main card switch is effected from the top of the deck in this video and accordingly, the set up is different.