input: Write a function to create a Python class which represents a 3D Vector. The class should have an __init__ method to initialize the x, y, and z components of the vector and a __str__ method to return the vector as a string in the format "(x, y, z)".
  
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(-1, -2, -3))=="(-1, -2, -3)"
  assert str(Vector3D(0, 0, 0))=="(0, 0, 0)"
  assert str(Vector3D(5.5, 3.14, 2.71))=="(5.5, 3.14, 2.71)"
  assert str(Vector3D(1, 1, 1))=="(1, 1, 1)"
output: 
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  