MATLAB: How to do matrix addition of different sizes

matrix addition

E = diag([12:4:56])

B=rand(12,3)

C=A+B

C=A
C(:,1:3)=A(:,1:3)+B(:,1:3)

Read about diag.

A = -4*ones(9,1) ; 
B = [1 1 0 1 1 0 1 1] ; 
C = ones(1,6) ; 
iwant = diag(C,-3)+ diag(B,-1) + diag(A) +diag(B,1) + diag(C,3)

B = [ones(1,3)*A(1); A; ones(1,3)*A(1)]