input: Write a python function to concatenate three lists.
  assert concatenate_lists([2, 3, 4], [5, 6, 7], [8, 9, 10]) == [2, 3, 4, 5, 6, 7, 8, 9, 10]
  assert concatenate_lists([1, 2], [3, 4], [5, 6]) == [1, 2, 3, 4, 5, 6]
  assert concatenate_lists([], [], [1, 2, 3]) == [1, 2, 3]
output: def concatenate_lists(list1, list2, list3):
      return list1 + list2 + list3