input: def sort_every_third_element(lst: list):
      """This function accepts a list and returns a new list with elements at positions
      that are multiples of three sorted, while leaving the rest of the list unchanged.
      >>> sort_every_third_element([1, 2, 3])
      [1, 2, 3]
      >>> sort_every_third_element([5, 6, 3, 4, 8, 9, 2])
      [2, 6, 3, 4, 8, 9, 5]
      """
output:     if len(lst) < 3:
          return lst
      
      # Extract every third element
      third_elements = lst[2::3]
      # Sort these elements
      third_elements_sorted = sorted(third_elements)
      
      # Write a copy of the input list to modify
      result = lst[:]
      
      # Replace every third element with the sorted values
      third_index = 0
      for i in range(2, len(lst), 3):
          result[i] = third_elements_sorted[third_index]
          third_index += 1
      
      return result