Which are strong acids?

(P) $\begin{array}{r}{\mathrm{HClO}}_3\end{array}$      (Q)  ${\mathrm{H}}_2{\mathrm{SeO}}_3$

(R) ${\mathrm{H}}_3{\mathrm{AsO}}_4$

Strong acids are those with weak O-H bonds. The core metal atom's oxidation number will grow as the number of oxygen atoms immediately linked to it rises, weakening the O-H bond as a result. Thus,  $\begin{array}{r}{\mathrm{HClO}}_3\end{array}$ is a strong acid.