Rolling dices possibility I must solve given task:
Three dice are rolled. What is possibility the sum of it to be $5$?
I guess I have to use combination formula, but I do not know the basic approach. 
Can you please explain. My thoughts:
Sum of $5$ can be only get from numbers: $1$, $2$ and $3$. - $221$ $113$. I guess those are permutations. How must I continue?

There are exactly three ways to order the triplet $2,2,1$ : $(2,2,1),(2,1,2),(1,2,2)$
Similarly there are three different ways to get $1,1,3$
At the end there are $6$ different ways to get $5$ as sum among the $6^3$ possibilities
$$P(X_1+X_2+X_3=5)=\frac{6}{6^3}=\frac{1}{6^2}$$
