I'm not really sure what this is supposed to mean.
%(x+y+z):3=value16
What guarantees that the sum of x,y,z is less than 3?
I guess you can start with this:
% test array
A = randi(9,21,6)
% get all xyz, sum
xyz = sum((A(16:20,4:6) - A(16:20,1:3))/12,2)
% expand and take mean
mn = zeros(size(xyz));
for r = 1:size(xyz,1)
mn(r) = mean(xyz(r):3);
end
mn
