The most stable oxidation state of Iron is

${}_{26}\mathrm{Fe}\to \left[\mathrm{Ar}\right]3{ \mathrm{d}}^{6}4{ \mathrm{s}}^2$

${{}_{26}\mathrm{Fe}}^{2+}\to \left[\mathrm{Ar}\right]3{ \mathrm{d}}^6$

${{}_{26}\mathrm{Fe}}^{3+}\to \left[\mathrm{Ar}\right]3{ \mathrm{d}}^5$

Due to the stable half filled configuration, The most stable oxidation state is +3,