The number of combinations that will result in a sum of 7 is:
1 + 6
6 + 1
2 + 5
5 + 2
3 + 4
4 + 3
For a total of 6 combinations per roll. While the total number of combinations per roll are:
6 * 6 = 36
Hence the probability that the sum is 7 would be:
Probability = 6 / 36 = 1 / 6
For a succession of three times, the overall probability that each three rolls will be a sum of 7 is:
Overall probability = (1/6) * (1/6) * (1/6)
Overall probability = 1/216 = 4.63 x 10^-3 = 0.463%
