input: Write a function to find all combinations of three elements from a list.
  assert comb3([1, 2, 3, 4]) == [[1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]]
  assert comb3([1, 2, 3, 4, 5]) == [[1, 2, 3], [1, 2, 4], [1, 2, 5], [1, 3, 4], [1, 3, 5], [1, 4, 5], [2, 3, 4], [2, 3, 5], [2, 4, 5], [3, 4, 5]]
  assert comb3([5, 6, 7, 8, 9]) == [[5, 6, 7], [5, 6, 8], [5, 6, 9], [5, 7, 8], [5, 7, 9], [5, 8, 9], [6, 7, 8], [6, 7, 9], [6, 8, 9], [7, 8, 9]]
output: def comb3(lst):
      from itertools import combinations
      return [list(c) for c in combinations(lst, 3)]