Oxidation states af the metal in the minerals haematite and magnetite, respectively, are: 

(i) in haematite $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right),$ oxidation number of $\mathrm{Fe}\left(\mathrm{x}\right)$ is: 

      $2x+3(-2)=0;2x=+6;x=6/2=+3$

(ii) Magnetite $\left(F{e}_3{O}_4\right)$ is an equimolar mixture of $\mathrm{FeO}$ and ${\mathrm{Fe}}_2{\mathrm{O}}_3$. 

Thus, oxidation number of $\mathrm{Fe}\left(\mathrm{x}\right)$ is$\mathrm{FeO}$ is:

$\mathrm{x}+1\mathrm{x}(-2)=0; \mathrm{x}=+2.$

 The oxidation number of $\mathrm{Fe}\left(\mathrm{x}\right)$

 in ${\mathrm{Fe}}_2{\mathrm{O}}_3$ is $+ 3$ (see ii above).