${\mathrm{KMnO}}_4$ acts as an oxidising agent. In neutral or faintly alkaline medium alkaline ${\mathrm{KMnO}}_4$ is reduced to ${\mathrm{MnO}}_2$.

$8{\mathrm{MnO}}_4^{-}+3{\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}\to 8{\mathrm{MnO}}_2+6{\mathrm{SO}}_4^{2-}+2{\mathrm{OH}}^{-}$

The change in oxidation state of manganese in this reaction is from $+7$ to $+4$. So,  In this reaction overall change in oxidation state of manganese will be 3.