The  $E^0$ value of  $M{n}^{3+}/M{n}^{2+}$ couple is much more positive than that of  $C{r}^{3+}/C{r}^{2+}$  or  $F{e}^{3+}/F{e}^{2+}$  couple.
 $$\begin{gathered} M{n}^{3+}/M{n}^{2+}{E}^0=1.57v; \\ C{r}^{3+}/C{r}^{2+}{E}^0=-0.41v; \\ F{e}^{3+}/F{e}^{2+}{E}^0=0.77v \end{gathered}$$
This is because

$3d^3$ config of  $C{r}^{3+}$  is stable due to half- filled $t_{2g}3$  config than $3d^4$ config of  $C{r}^{2+}$  having ${t_{2g}}^{3}e{g}^1$ config

$3d^5$ config of  $F{e}^{3+}$ is stable than  $3d^6$ config of  $F{e}^{3+}$

The  $3d^5$ config of  $M{n}^{2+}$  is stable than $3d^4$ config of  $M{n}^{3+}$  

Sum of the three ionization energies $(I_1+I_2+I_3)$ for chromium is very high compared than those of Mn and Fe