In 1, there are 6 outcomes for each die, so for three dice, the total combination is 6 x 6 x 6 = 216 outcomes. Hence, the probability of any individual outcome is 1/216
The outcomes that will add up to 6 are
1+1+4
1+4+1
4+1+1
1+2+3
1+3+2
2+1+3
2+3+1
3+1+2
3+2+1
2+2+2
Hence the probability is 10/216
In 3, the minimum sum of the three dice is 3. so we start with this
P(n = 3)
1+1+1 ; 1/216
P(n = 4)
1+1+2
1+2+1
2+1+1 ; 3/216
P(n = 5)
1+1+3
1+3+1
3+1+1
1+2+2
2+1+2
2+2+1; 6/216
The sum in 3 is 10/216 or 5/108