Acidic character of oxides in group 15 to 18 elements decreases down the group. As we move down the group, the atomic size increases and electronegativity decreases. The oxide in the higher oxidation state of the elements is more acidic than that of lower oxidation state. In $\mathrm{N}_2 \mathrm{O}_3(\mathrm{I}), \mathrm{P}_2 \mathrm{O}_3$ (II) and $\mathrm{As}_2 \mathrm{O}_3$ (IV), N, $P$ and As are in +3 oxidation state respectively whereas in $\mathrm{N}_2 \mathrm{O}_5$ (III) the $\mathrm{N}$ is in +5 oxidation state. So, the acidity order is
$$
\mathrm{N}_2 \mathrm{O}_5>\mathrm{N}_2 \mathrm{O}_3>\mathrm{P}_2 \mathrm{O}_3>\mathrm{As}_2 \mathrm{O}_3 \text {, i.e. } \mathrm{III}>\mathrm{I}>\mathrm{II}>\mathrm{IV}
$$