The cards right next to the 'random' cards are shown. Amazingly, the spectator has found the matching mate cards! First surprise!
The magician now turns the face down 'mystery' card to reveal an Ace. The top card of the middle packet and the two outer packets are flipped over to reveal the other three Aces!! Second surprise!!
Finally, you come clean and explain that this amazing effect was made possible by using a marked deck.
But, you say that the cards are not marked on the back but on the faces. With this statement, you spread all the remaining cards face up to show that all the cards are the Three of Clubs!!! Third surprise!!!