Given 3 dices are rolled, what is the probability of the sum of the faces is 6?

If n(A) is the number of elementary events in A

$\displaystyle n(A)=_3C_1+_3P_3+_3C_0$

How do you get that?

P.S. How do I properly type permutations in latex? The first number is a bit far to the letter.