input: Write a function to find the third largest element in a given list. If the length of the list is less than 3, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([1, 2, 3, 4, 5])==3
  assert third_largest([7, 7, 7])==None
  assert third_largest([9, 3, 8, 1])==3
  assert third_largest([6])==None
output: def third_largest(lst):
      if len(lst) < 3:
          return None
      lst = sorted(set(lst), reverse=True)
      return lst[2] if len(lst) > 2 else None