Step 1: Aniline with $\mathrm{B}{\mathrm{r}}_2$ in water gives $2,4,6$-tribromoaniline ($A$), since the strongly activating $-\mathrm{N}{\mathrm{H}}_2$ directs Br to the ortho and para positions.

Step 2: $\mathrm{NaN}{\mathrm{O}}_2/HCl$ at $273-278$ K diazotises the remaining $-\mathrm{N}{\mathrm{H}}_2$, giving the aryldiazonium chloride $B$ of $2,4,6$-tribromoaniline.

Step 3: Treatment with $\mathrm{HB}{\mathrm{F}}_4$ converts the diazonium chloride into the fluoroborate salt. Subsequent reaction with $\mathrm{NaN}{\mathrm{O}}_2/Cu$ on heating is a variant of the Sandmeyer-style nitration: the $-{\mathrm{N}}_2^{+}$ group is replaced by $-\mathrm{N}{\mathrm{O}}_2$.

The product is the original tribromo arene with $\mathrm{N}{\mathrm{O}}_2$ where the amino group used to be: $1,3,5$-tribromo-$2$-nitrobenzene.