Sentence:  In a <WSD>trifecta</WSD>, you must wager on three horses: the one that finishes first (win), the one that finishes second (place), and the one that finishes third (show) in order. 
Meaning of the word marked with <WSD> tags:  A bet in which the bettor must select the first three placegetters of a race in the order in which they finish.