There are 13 diamonds in the deck, so [tex]\dbinom{13}3[/tex] ways of drawing 3 diamonds.
There is a total of [tex]\dbinom{52}3[/tex] ways of drawing any 3 cards from the deck.
So the probability of drawing 3 diamonds is
[tex]\dfrac{\binom{13}3}{\binom{52}3}=\dfrac{11}{850}\approx0.012941[/tex]
where [tex]\dbinom nk=\dfrac{n!}{k!(n-k)!}[/tex].
