Poisson Equation 3D

I have to solve the poisson equation in 3D with BC Dirichelet in Matlab. I have a problem with the numeric solution. The fault to the numeric solution is high. Increment is in every direction different.

I want to fill in the BC in to my rigth hand side. The M is the number of points in the grid. hx, hy and hx are the increment. Is their any mistake in my right hand side when i want to fill in BC? I checked my array and their is nothing wrong with it.

for k=1:Mz

for l=1:Mx

IndexT=Mx*(My-1)+l+(k-1)*Mx*My;

IndexB=l+(k-1)*Mx*My;

rhs(IndexT)=rhs(IndexT)-(uT(l*hx,k*hz)*By/(hy^2)); top edge

rhs(IndexB)=rhs(IndexB)-(uB(l*hx,k*hz)*By/(hy^2)); bottom edge

end

for p=1:My

IndexL=1+(p-1)*Mx+(k-1)*Mx*My;

IndexR=Mx+(p-1)*Mx+(k-1)*Mx*My;

rhs(IndexL)=rhs(IndexL)-(uL(p*hy,k*hz)*Ax/(hx^2)); left edge

rhs(IndexR)=rhs(IndexR)-(uR(p*hy,k*hz)*Ax/(hx^2)); right edge

end

end

i=0;

for k=1:My

for l=1:Mx

IndexF=MxMyMz-Mx*My+i+1;

IndexH=i+1;

rhs(IndexF)=rhs(IndexF)-(uF(l*hx,k*hy)*Cz/(hz^2)); front edge

rhs(IndexH)=rhs(IndexH)-(uH(l*hx,k*hy)*Cz/(hz^2)); back edge

i=i+1;

end

end