How to to add column 3rd+4th, 6th+7th and 9th+10th from a 8x12 matrix?

I want to add column 3rd+4th, 6th+7th and 9th+10th from the matrix B. I want to apply the rule for a 2D sparse matrix.

A = [1,2,3;4,5,6]

A = 2×3

1 2 3 4 5 6

N=4;
B = kron(eye(N),A)

B = 8×12

1 2 3 0 0 0 0 0 0 0 0 0 4 5 6 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 0 0 0 4 5 6 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 0 0 0 4 5 6 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 0 0 0 4 5 6

Answer 1

[Jan]

C1 = B(:, 3) + B(:, 4);
C2 = B(:, 6) + B(:, 7);
C3 = B(:, 9) + B(:, 10);

Or maybe:

C = B(:, [3,6,9]) + B(:, [4,7,10])