input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer to set the x, y, and z components. Additionally, it should have a method to return a string representation of the vector in the format "(x, y, z)".  
  
  assert Vector3D(1, 2, 3).__str__() == "(1, 2, 3)"  
  assert Vector3D(0, 0, 0).__str__() == "(0, 0, 0)"  
  assert Vector3D(-1, 5, 3).__str__() == "(-1, 5, 3)"  
  assert Vector3D(10, -2, 7).__str__() == "(10, -2, 7)"  
  assert Vector3D(4, 4, 4).__str__() == "(4, 4, 4)"
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return f"({self.x}, {self.y}, {self.z})"