Step 1:
We can rewrite the limit as follows:
$$
\lim _{h \rightarrow 0} \frac{\sin (a+3 h)-3 \sin (a+2 h)+3 \sin (a+h)-\sin a}{h^{3}} = \lim _{h \rightarrow 0} \frac{\cos(a+\frac{3h}{2}) \cdot 2\sin(\frac{3h}{2}) - 3\cos(a+\frac{3h}{2}) \cdot
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