The electronic configurations of ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ is [Ar] 3d5 4s2 and [Ar]3d4 

Because Cr2+ gains Cr3+, which has a more stable  half-filled structure, after losing one electron, Hence,Cr2+ is a powerful reducing agent

${\mathrm{Cr}}^{2+}⟶{\mathrm{Cr}}^{3+}+e^{-}$

Mn3+, on the other hand, works as an oxidising agent because the reduction of Mn3+ to Mn2+ produces the half-filled (d5) structure, which has more stability.

${\mathrm{Mn}}^{3+}+e^{-}⟶{\mathrm{Mn}}^{2+}$