input: Write a python function to create a 3D point and return its string representation.
  assert Point3D(1, 2, 3).to_string() == "(1, 2, 3)"
  assert Point3D(0, 0, 0).to_string() == "(0, 0, 0)"
  assert Point3D(-1, -2, -3).to_string() == "(-1, -2, -3)"
output: class Point3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def to_string(self):
          return f"({self.x}, {self.y}, {self.z})"