input: Write a function to return the third largest element in a given list of integers. If the list has fewer than three unique elements, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([1, 2])==None
  assert third_largest([5, 5, 4, 4, 3, 3])==4
  assert third_largest([10, 20, 30, 40, 50])==30
  assert third_largest([100, 200])==None
output: def third_largest(lst):
      unique_elements = list(set(lst))
      if len(unique_elements) < 3:
          return None
      unique_elements.sort(reverse=True)
      return unique_elements[2]