Standard electrode potentials are 
$\begin{array}{l}{\mathrm{Fe}}^{2+}/\mathrm{Fe},{\mathrm{E}}^0=-0.44\mathrm{V}\\ {\mathrm{Fe}}^{3+}/{\mathrm{Fe}}^{2+},{\mathrm{E}}^0=+0.77\mathrm{V}\end{array}$
${\mathrm{Fe}}^{2+},{\mathrm{Fe}}^{3+}$ and Fe blocks are kept together, then

Conversion of Fe²⁺ to Fe is non-spontaneous while converting Fe³⁺  to Fe²⁺ is spontaneous.
So, Fe³⁺  will be spontaneously converted to Fe²⁺ . So, Fe³⁺  will decrease.