To help your understanding I suggest you write out the sample space i.e.

(1,1,1) (1,1,2) (1,1,3)

(1,2,1) (1,2,2) (1,2,3)

(1,3,1) (1,3,2) (1,3,3)

(2,1,1) (2,1,2) (2,1,3)

....

etc.

This is all 27 possibilities and sum each, then find the probability of each sum.