How many three of diamonds are in a deck of 52 cards?

Of the 52 cards, there are an equal number of each of the 4 suits (Clubs, Spades, Diamonds and Hearts) so there are 52/4 = 13 of each suit.

In each suit there is an A, 2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K = 13 cards

Out of the 13 Diamond cards, there is only one which is a 3.

So there is only one 3 of diamonds in a standard deck of 52