If I understand correctly, you are rolling 10 dice and you want to calculate the probability that:
a) exactly three 6's are showing.
b) at least three 6's are showing.
a) We need to know how many ways there are to choose 3 from 10:
We also need to know the probability of getting one particular set of 3 dice to show 6's:
Hence, the probability in question is:
b) Here, it will be simpler to calculate the probability of getting less than three 6's, i.e., to get 0, 1, or 2 6's, then subtract this from 1 (Do you understand why?). See if you can use the same technique outlined in part a) to find this probability.