input: Write a python function to create a new vector by adding two 3D vectors and multiplying a vector by a scalar.
  assert (Vector3D(1, 2, 3) + Vector3D(4, 5, 6)) == Vector3D(5, 7, 9)
  assert (Vector3D(1, 2, 3) * 3) == Vector3D(3, 6, 9)
  assert (Vector3D(0, 0, 0) + Vector3D(1, 1, 1)) == Vector3D(1, 1, 1)
  assert (Vector3D(2, 3, 4) * 0) == Vector3D(0, 0, 0)
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
  
      def __add__(self, other):
          return Vector3D(self.x + other.x, self.y + other.y, self.z + other.z)
  
      def __mul__(self, scalar):
          return Vector3D(self.x * scalar, self.y * scalar, self.z * scalar)
  
      def __eq__(self, other):
          return self.x == other.x and self.y == other.y and self.z == other.z