$$ \mathrm{E}_{\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}}^0>\mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}}^{\circ} \text { or } \mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}}^{\circ} $$

Electronic configuration of $$\mathrm{Mn}^{3+}=[\mathrm{Ar}] 3 d^4$$

Electronic configuration of $$\mathrm{Mn}^{2+}=[\mathrm{Ar}] 3 d^5$$

Electronic configuration of $$\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 d^3$$

Electronic configuration of $$\mathrm{Cr}^{2+}=[\mathrm{Ar}] 3 d^4$$

As $\mathrm{Mn}^{3+}$ from $d^4$ configuration goes to more stable $d^5$ configuration (Half filled), due to more exchange energy in $d^5$ configuration.