input: Write a function to find the third largest element in a given list. If the list has fewer than three elements, the function should return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([3, 2, 6])==2
  assert third_largest([5, 5, 5])==None
  assert third_largest([7])==None
  assert third_largest([8, 9, 10, 10])==9
output: def third_largest(lst):
      if len(lst) < 3:
          return None
      unique_lst = list(set(lst))
      unique_lst.sort(reverse=True)
      if len(unique_lst) < 3:
          return None
      return unique_lst[2]