input: Write a function to create a Python class which represents a 3D Vector. The class should have an `__init__` method that initializes the vector with three coordinates (x, y, z) and a `__str__` method that returns the vector in a string format like "(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(4, 5, 6)) == "(4, 5, 6)"
  assert str(Vector3D(10, 20, 30)) == "(10, 20, 30)"
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) + ")"