input: def third_largest(lst):
      """
      Given a list of numbers, this function returns the third largest element.
      If the list has fewer than 3 distinct elements, it returns None.
      >>> third_largest([12, 13, 13, 45, 22, 99])
      22
      >>> third_largest([1, 2, 3, 4, 5])
      3
      >>> third_largest([10, 10, 10, 10])
      None
      """
output:     if len(lst) < 3:
          return None
      
      unique_numbers = list(set(lst))
      unique_numbers.sort(reverse=True)
      
      if len(unique_numbers) < 3:
          return None
      
      return unique_numbers[2]