How to concatenate 3 RGB channels without using the cat(3,R,G,B) function [closed]

As mentioned by @Pascal, the most straightforward solution is

new = intmax('uint8') - A;

If you insist on using loops (which I highly advise against in this case), these should work:

[r,c,s] = size(A);
new = zeros(r,c,s,'uint8'); % alternatively: zeros(r,c,s,'like',A);
for iR = 1:r % the middle :1: is implied
  for iC = 1:c
    for iS = 1:s % s stands for "slice"
      new(iR, iC, iS) = intmax('uint8') - A(iR, iC, iS);
    end
  end
end

for iR = 1:r
  for iC = 1:c
    new(iR, iC, 1) = intmax('uint8') - R(iR, iC);
    new(iR, iC, 2) = intmax('uint8') - G(iR, iC);
    new(iR, iC, 3) = intmax('uint8') - B(iR, iC);
  end
end

As you can see, all of the above solutions do not use the cat function which you wanted to avoid.