Answer
$A$, $B$ and $C$ being 3 angles of a triangle means that the sum of them equals $180^\circ$.
Therefore, $\sin(A+B+C)$ equals $\sin180^\circ$, which equals $0$.
Work Step by Step
If $A$, $B$ and $C$ are the 3 angles of a triangle, that means
$$A+B+C=180^\circ$$
Therefore, $$\sin(A+B+C)=\sin180^\circ=0$$
