(* Cube of side length 1, with upper surface at V=1 and all other surfaces at V=0 *)

V[x_,y_,z_]=Sum[16* 1/(Pi^2 n m Sinh[Pi Sqrt[n^2 + m^2]]) Sin[n Pi x/1] Sin[m Pi y/1] Sinh[Pi z Sqrt[n^2 + m^2]/1],{n,1,9,2},{m,1,9,2}]

(* Solution depicted via cross-sectional cuts at z = 0.9, 0.6, and 0.4. *)
(* first cut: z = 0.9 *)
plane1[x_,y_]=V[x,y,.9];
Plot3D[plane1[x,y],{x,0,1},{y,0,1},PlotRange->{0,1}]

(* 2nd cut: z = 0.6 *)
plane3[x_,y_]=V[x,y,.6];
Plot3D[plane3[x,y],{x,0,1},{y,0,1},PlotRange->{0,1}]

(* 3rd cut: z = 0.4 *)
plane4[x_,y_]=V[x,y,.4];
Plot3D[plane4[x,y],{x,0,1},{y,0,1},PlotRange->{0,1}]

(* Alternate method: use a "heat map" to display the cuts. This shows z = 0.1, ..., 0.9. *)
planez[x_,y_,z_]=V[x,y,z];
Table[DensityPlot[planez[x,y,z],{x,0,1},{y,0,1},PlotRange->{0,1},  PlotPoints->25,ColorFunction->(Blend[{Blue,Purple,Red},#]&)], {z,0,0.9,.1}]