One of my friend Nhat Quy's routines. Show spectator(s) 3 Blank cards and ask them to pick 1.

The magician picks out 4 random cards and sandwiches 3 blank cards in between. With a magical move, 3 blank cards turn into 3 of a kind with the selected card.

Kicker ending: 4 random cards turn into 4 Aces.