The first person in the group can be selected in any one of 8 ways.
Once selected, the second person can be selected in any one of 7 ways.
Once the first two are selected, the remaining person can be selected in any one of 6 ways.
This can be done in a total of 8 x 7 x 6 ways.
But, since the order in which the people appear in the group is not important, and since 3 people can be arranged in 3 x 2 x 1 = 6 ways, you need to divide the above number by 6 to filter out those groups with the same members (just rearranged). The answer is therefore,
(8 x 7 x 6) / (3 x 2 x 1) = 56
