a=1;
b=1;
V0=1;
c1=1;
c2=1;
c3=-1;
Plot3D[V0(c1 Sin[π x/a]Sinh[π y/b]/Sinh[π]+c2 Sin[2π x/a]Sinh[2π y/b]/Sinh[2π]+c3 Sin[3π x/a]Sinh[3π y/b]/Sinh[3π]),{x,0,a},{y,b/2,b},PlotRange->All,AxesLabel->Automatic]
Plot[Table[V0(c1 Sin[π x/a]Sinh[π y/b]/Sinh[π]+c2 Sin[2π x/a]Sinh[2π y/b]/Sinh[2π]+c3 Sin[3π x/a]Sinh[3π y/b]/Sinh[3π]),{y,0,b,b/10}],{x,0,a},PlotRange->All,AxesLabel->Automatic]
Plot[Table[V0(c1 Sin[π x/a]Sinh[π y/b]/Sinh[π]+c2 Sin[2π x/a]Sinh[2π y/b]/Sinh[2π]+c3 Sin[3π x/a]Sinh[3π y/b]/Sinh[3π]),{y,0,b,b/10}],{x,0,a},PlotRange->{0,.1},AxesLabel->Automatic]
Plot[Table[V0(c1 Sin[π x/a]Sinh[π y/b]/Sinh[π]+c2 Sin[2π x/a]Sinh[2π y/b]/Sinh[2π]+c3 Sin[3π x/a]Sinh[3π y/b]/Sinh[3π]),{x,0,a,a/4}],{y,0,b},AxesLabel->Automatic]

Plot3D[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,5,2}]),{x,0,a},{y,0,b},PlotRange->All,AxesLabel->Automatic]
Plot3D[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,11,2}]),{x,0,a},{y,0,b},PlotRange->All,AxesLabel->Automatic]
Plot3D[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,21,2}]),{x,0,a},{y,0,b},PlotRange->All,AxesLabel->Automatic]
Plot3D[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,1001,2}]),{x,0,a},{y,0,b},PlotRange->All,AxesLabel->Automatic]
Plot[Table[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,1001,2}]),{y,0,b,b/10}],{x,0,a},PlotRange->All,AxesLabel->Automatic]
Plot[Table[V0(Sum[4/(n π)Sin[n π x/a]Sinh[n π y/b]/Sinh[n π],{n,1,1001,2}]),{y,0,b,b/10}],{x,0,a},PlotRange->{0,.1},AxesLabel->Automatic]