In poker, a flush is a 5-card hand where all the cards have the same suit. Recall that a deck of cards has 52 cards. There are 4 suits (hearts, clubs, diamonds and spades) and each suit has 13 cards.
How many ways can you get a flush of hearts, diamonds or spades?
For this question, I am thinking the answer is 13C5+13C5+13C5 where C is choses.I am not so sure because it looks tricky
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It is exactly as you wrote: $13\choose5$ counts the number of different ways to get 5 cards out of that particular suit. This is the same for any suit, and so if you're interested in those particular flushes, the answer is as you gave: $3\cdot {13\choose 5}$.