input: from typing import List
  
  def vector3d_operations(vectors: List[Vector3D]) -> List[str]:
      """ Given a list of Vector3D instances, return a list of strings in the
      format (x, y, z).
      >>> vector3d_operations([Vector3D(1, 2, 3), Vector3D(4, 5, 6)])
      ['(1, 2, 3)', '(4, 5, 6)']
      >>> vector3d_operations([Vector3D(0, 0, 0), Vector3D(-1, 1, -1)])
      ['(0, 0, 0)', '(-1, 1, -1)']
      """
output:     result = []
      for vector in vectors:
          result.append(f"({vector.x}, {vector.y}, {vector.z})")
      return result