Answer
The graphs do not appear to coincide.
Work Step by Step
Please note that the graphs of $y=\sin x +\sin 2x$ (the red graph) and $y=\sin 3x$ (the blue graph) do not coincide.
For $x=\frac{\pi }{2}$ we have$$\sin \frac{\pi }{2} + \sin 2(\frac{\pi }{2})=1 \neq -1 = \sin \frac{3\pi }{2}.$$
