Three-card monte, also called "Find the Lady," typically involves a dealer challenging passers-by to pick the queen out of three face-down playing cards "randomly" shuffled out on a cardboard box. The dealer often uses sleight of hand and misdirection to rig the game in his or her favor. In addition, the dealer has accomplices in the crowd to help with encouraging victims to play and to help misdirection.
A shell game is the same type of operation, using cups and a ball instead of playing cards. Again, sleight of hand is used to tip the game in the dealer's favor.
When a victim catches on to the plot, accomplices will "spot" police nearby and the dealer will quickly fold up the operation and depart. Hence the common use of cardboard boxes, which can be carried away or abandoned as needed, as playing tables.