input: Write a function to get the third largest element in a given list. If the list has fewer than three elements, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([10, 20, 40, 30, 50])==30
  assert third_largest([5, 5, 5, 5, 5, 5])==None
  assert third_largest([7])==None
  assert third_largest([])==None
output: def third_largest(lst):
      if len(lst) < 3:
          return None
      unique_sorted = sorted(set(lst), reverse=True)
      if len(unique_sorted) < 3:
          return None
      return unique_sorted[2]