input: Write a function to create a Python class representing a 3D Vector with an initializer to store the coordinates (x, y, z) and a method to return a string representation of the vector.
  
  
  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) + ")"
  
  # Test cases
  assert str(Vector3D(1, 2, 3)) == "(1, 2, 3)"
  assert str(Vector3D(0, 0, 0)) == "(0, 0, 0)"
  assert str(Vector3D(-1, 5, -7)) == "(-1, 5, -7)"
  assert str(Vector3D(3.5, -2.1, 4.9)) == "(3.5, -2.1, 4.9)"
  assert str(Vector3D(-3, -4, -5)) == "(-3, -4, -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) + ")"