Why is the Eº values for Mn³⁺/Mn²⁺ couple much more positive than for Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺ ?

The large positive Eº value for Mn³⁺/Mn²⁺ shows that Mn²⁺ is much more stable than Mn³⁺. This is due to the fact that Mn²⁺ has the configuration 3d⁵ which is half-filled and hence is very stable. Thus, the third ionization energy of Mn will be very high. In fact, this is the reason that +3 state of Mn is of little importance.