Standard electrode potentials are ${\mathrm{Fe}}^{2+}/\mathrm{Fe}\left({\mathrm{E}}^0=-0.44\right);{\mathrm{Fe}}^{3+}/{\mathrm{Fe}}^{2+}\left({\mathrm{E}}^0=0.77\right)$ ${\mathrm{Fe}}^{2+},{\mathrm{Fe}}^{3+}$ and Fe blocks are kept together. Then 

The metals having higher negative values of their electrode potential can displace metal having lower values from their salt solution

The reactions taking place are ; 

$Fe\left|F{e}^{2+\hspace{0.17em}}\right||\hspace{0.17em}F{e}^{3+ }|F{e}^{2+}$

$E_{cell}^o=(+0.44) +\hspace{0.17em}(0.77)\hspace{0.17em}= +1.21V\hspace{0.17em}$

celll reaction is possible. 
Hence, the magnitude of Fe³⁺ ions will decrease.