input: Write a function to find the third largest element in a given list. If the list has fewer than three unique elements, the function should return None.
  
  assert third_largest([12, 13, 13, 45, 22, 99]) == 22
  assert third_largest([3, 4, 4, 5, 1]) == 3
  assert third_largest([1, 1, 1]) == None
  assert third_largest([7, 3, 4, 5]) == 4
  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]