How do I incorporate multiple values of the Z-coordinate into my calculation of projection area?

Hello, I have a matrix A that consists of the coordinates of the 8 corners of 3 cubes, for a total of 24 points. I first import these coordinates in X,Y,Z matrices and then I want to plot and calculate the area of the projection on the X-Z plane. However, due to overlapping X and Y coordinates, half of the Z values are ignored. How do I incorporate multiple values of Z into my calculation of projection area?

 clear all
close all
clc
A=[-100  -40  50
-100  -40  5
-80  -40  50
-80  -40  5
-100  5  5
-100  5  50
-80  5  50
-80  5  5
-100  0  50
-100  0  5
20  0  5
20  0  50
-100  50  5
-100  50  5
20  50  50
20  50  5
-30  70  50
-30  70  5
5  70  5
5  70  50
-30  120  50
-30  120  5
5  120  50
5  120  5]; %3 cubes Coordinate Matrix
[X,Y] = meshgrid(-150:1:150,-150:1:150);
Z=zeros(size(X,1),size(X,2));
for i=1:length(A)
    [p,j]=find(X==A(i,1));
    [k,l]=find(Y==A(i,2));
    Z(k(1),j(1))=A(i,3);
end
plot(X(1,:),max(Z))
hold on
plot(X(1,:),min(Z),'r')
polyarea(X(1,:),max(Z))

The above is the code I am using