When three moles of ozone completely reacts with ${\mathrm{SO}}_2$, the number of moles of oxygen formed is

Ozone reacts with ${\mathrm{SO}}_2$ to form ${\mathrm{SO}}_3$.

${\mathrm{SO}}_2+{\mathrm{O}}_3\to {\mathrm{SO}}_3$

On balancing this equation we get,

$9{\mathrm{SO}}_2+3{\mathrm{O}}_3\to 9{\mathrm{SO}}_3$

As there is no oxygen formed here, the number of moles of oxygen formed is zero.