D=[A B C];
S = numel(B);
xx = reshape(B(1:S - mod(S, 3)), 3, );
BB = sum(xx, 1).' / 3;
F=[A B C BB];
You will notice F is not possible because BB has just 10 variables (Dimensions of arrays being concatenated are not consistent.)
What I need to do is make each value in BB constant for 3 rows so that F can become possible.