Writing a loop to calculate sums horizontally for ALL rows

I have a matrix that's n-by-12. I have the following loop to calculate the sums for the values of specific cell groups within this array. However, this loop only works for the first row. I'm not sure how to incorporate that it should do this for ALL rows, however many there may be.

Say the array looks like:

row 1: [96, 24, 58, 29, 31, 49, 37, 63, 3, 14, 14, 13]
row 2: [45, 90, 36, 58, 56, 78, 46, 3, 89, 354, 566, 463]
etc.

The number of rows will vary, so that needs to be accounted for. Also happy to make modifications if there is a cleaner way to write the whole thing.

             for k = 1:length(a)
                  c = sum(a(:,1)+a(:,2)+a(:,3));
                  c(end+1) = sum(a(:,4)+a(:,5)+a(:,6));
                  c(end+1) = sum(a(:,7)+a(:,8)+a(:,9));
                  c(end+1) = sum(a(:,10)+a(:,11)+a(:,12));
              end

Answer 1

[Star Strider]

One approach:

n = 7;
A = rand(n, 12);                                        % Create Matrix
cv = sum(A);                                            % Sum Each Column
cm = reshape(cv, 3, []);                                % Create (3x4) Matrix Of Column Sums
c = sum(cm);                                            % Row Vector Of Desired Column & Row Sums

Experiment to get the result you want.