The term 'Casino Play' can be summarized as an activity involving betting. The three-letter word associated with this activity is 'BET'. Betting is a popular form of gambling in which players wager money on the outcome of a game or event. It is often found in casinos, where players can BET on a variety of games such as poker, blackjack, roulette, and slots. Betting is also popular in sports, where players can place bets on the outcome of a match or race.