According to Penn and Teller , their fondness for the three of clubs is because it shows up so well on television.

Additionally, some years ago (related in their 1997 book How to Play in Traffic and in an appearance on Late Night with Conan O'Brien) they bought themselves a memorial marker (cenotaph) at the Forest Lawn Memorial Park in the Hollywood Hills.

On it is:

PENN & TELLER (picture of the three of clubs) "IS THIS YOUR CARD?" CENOTAPH

The book includes both instructions on how to do the card trick (to "force" the person to pick the three of clubs) and a nice map to where the marker is. The idea is to appear to screw up the trick and then later show your friend the marker as you sightsee.

(Many famous Hollywood stars and celebrities are buried there including: Humphrey Bogart, George Burns and Gracie Allen, Lon Chaney, Larry Fine of "The Three Stooges," and Walt Disney who is not frozen as the legend tells it.)