input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer method to set the x, y, and z coordinates, and a string representation method to return the vector 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(10, 20, 30)) == '(10, 20, 30)'
  assert str(Vector3D(-5, 5, -5)) == '(-5, 5, -5)'
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) + ")"