$E_{F{e}^{3+}/F{e}^{2+}}^0=0.772V;E_{F{e}^{2+}/Fe}^0=-0.44V$
Then $E_{F{e}^{3+}/Fe}^0$ will be

$\begin{array}{l}F{e}^{3+}+e^{-}\stackrel{ }{\to }F{e}^{2+}{E}^0=+0.772V\mathrm{......}\left(1\right)\\ F{e}^{2+}+2e^{-}\stackrel{ }{\to }Fe{E}^0=+0.44V\mathrm{......}\left(2\right)\\ F{e}^{3+}+3e\stackrel{ }{\to }Fe{E}^0=?\\ \Delta G=\Delta G_1+\Delta G_2\end{array}$

-nE = -n₁E₁ -n₂E₂

nE = -n₁E_{1 +} n₂E₂

$E=\frac{1\left(0.772\right)+2\left(-0.44\right)}{3}=-0.036V$