In neutral or faintly alkaline medium, KMnO _ 4 being a powerful...

${KMnO}_{4}$ acts as an oxidising agent. In neutral or faintly alkaline medium alkaline ${KMnO}_{4}$ is reduced to ${MnO}_{2}$.

$8{MnO}_{4}^{-} + 3{S}_{2}{O}_{3}^{2 -} + {H}_{2}O \to 8{MnO}_{2} + 6{SO}_{4}^{2 -} + 2{OH}^{-}$

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.