$E_{cell}^o\, = \,3$

Given :

$E_{A{g^ + }/Ag}^o\, = \,x$        ....... $(1)$

$E_{F{e^{2 + }}/Fe}^o\, = \,y$    ....... $(2)$

$E_{F{e^{3 + }}/Fe}^o\, = \,Z$    ....... $(3)$

Using equation:

$\Delta {G^o}\, = \, - \,nF{E^o}$

$\Delta G_1^0\, = \, - \,Fx$

$\Delta G_2^0\, = \, - \,2Fy$

$\Delta G_3^0\, = \, - \,3Fz$

$F{e^{2 + }}\, + \,2{e^ - }\, \to \,Fe\, - \,2Fy$

$\mathop {F{e^{3 + }}}\limits_ -  \, + \,\mathop {3{e^ - }}\limits_ -  \, \to \,\mathop {Fe}\limits_ -  \, - \,\mathop {3Fz}\limits_ +  $

$\overline {F{e^{2 + }}\, \to \,F{e^{3 + }}\, + \,{e^ - }\,( - 2Fy\, + \,3Fz)} $

$\underline {A{g^ + }\, + \,{e^ - }\, \to \,Ag\, - \,Fx} $

$\Delta {G_{Total\,}}\, = \, - \,2\,Fy\, + \,3Fz\, - \,Fx\, = \, - \,FE_{cell}^o$

$E_{cell}^o\, = \,x\, + \,2y\, - \,3z$