input: Write a python function to add two Vector3 objects and return the resulting vector.
  assert str(Vector3(1, 2, 3) + Vector3(4, 5, 6)) == "x = 5, y = 7, z = 9"
  assert str(Vector3(-1, -2, -3) + Vector3(1, 2, 3)) == "x = 0, y = 0, z = 0"
  assert str(Vector3(0, 0, 0) + Vector3(0, 0, 0)) == "x = 0, y = 0, z = 0"
output: class Vector3:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
  
      def __add__(self, other):
          return Vector3(self.x + other.x, self.y + other.y, self.z + other.z)
  
      def __str__(self):
          return f"x = {self.x}, y = {self.y}, z = {self.z}"