Here $\overrightarrow{A} .{B}$ =$AB\, cos \theta$ = $AB\, cos 90^\circ$ = O $\Rightarrow$ $\overrightarrow{A} \perp \overrightarrow{B}$
Similarly, $\overrightarrow{A} \perp \overrightarrow{C} \Rightarrow \overrightarrow{B}$ and $ \overrightarrow{C}$ are in the same plane and
$ \overrightarrow{A}$ is perpendicular to them.
Thus $ \overrightarrow{A} || \overrightarrow{B} \times \overrightarrow{C}$