Let $$f(x) = \sin x + \sin 2x + \cos 3x$$. Then the graph of $$g(x) = \sin (x + \pi) + \sin (2x + \pi) + \cos (3x + \pi)$$:

Although option C might seem like a good choice,we have to remember that the given function is not algebraic is nature. 
Upon simplifying g(x) we get, 
$$g(x) = \sin (x + \pi) + \sin (2x + \pi) + \cos (3x + \pi)$$
$$g(x) = -\sin (x) - \sin (2x) - \cos (3x)$$
This is because $$\pi\ +x\ $$ lies in the third quadrant, where both sine and cosine functions are negative. 
We can get g(x) upon reflection of f(x) along the x-axis, but it is not possible using the operations given in option A, B and C. 
Therefore, Option D is the correct answer.