For M2+/M and M3+/M2+ systems the EV values for some metals are as follows

1)Reduction potential tells us the ease with which the Metal can get reduced,  As E° for Cr3+/Cr2+ is negative (–0.4 V), this means that Cr3+ ions in solution cannot be reduced to Cr2+ ions i.e., Cr3+ ions are very stable. As a further comparison of E° values show that Mn3+ ions more readily than Fe3+ ions which means that Mn3+ is least stable. 

So Stability of metal ions is as follows:

Mn3+<Fe3+<Cr3+.

2) Reduction potential tells us the ease with which the Metal can get reduced or the difficulty with which they are oxidized, As the reduction potential increases in the following order Mn2+/Mn < Cr2+/Cr< Fe2+/Fe

So oxidation of Fe is not as easy as Cr and Mn. So increasing order of the metals to get oxidised is as follows:

Fe<Cr<Mn