The question is somewhat ambiguous. One possible interpretation is that they are: {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3} and {1, 2, 3}.
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7 groups, with 3 in each3 groups, with 7 in each21 groups, with 1 in each1 group, with 21 in each
[1 3 2 7] [7 6] [6 3 4] [3 4 1 1 4] [1 4 8] [3 1 9] [9 2 2] 7
-7+3, or 3-7.
You can make three different groups of three out of nine. 9/3=3
7 + 7*3!/3 = 7 + 7*6/3 = 7 + 7*2 = 7 + 14 = 21