Number of ways of drawing 4 cards from 52 cards = 52C4
In a deck of 52 cards, there are 13 diamonds and 13 spades.
∴ Number of ways of drawing 3 diamonds and one spade = 13C3 x 13C1
Thus, the probability of obtaining 3 diamonds and one spade =13C3 x 13C1 / 52C4