input: Write a function to get the third largest element in a given list. If the list does not contain at least three distinct elements, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([1, 2, 2, 3])==1
  assert third_largest([10, 10, 10])==None
  assert third_largest([1])==None
  assert third_largest([])==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]