Adding matrix diagonally

Hi I have a number of 3x3 matrix which i want to combine into one big matrix. But I need to add the end of the first matrix with the start of the second matrix, the end of the second matrix with the start of the third matrix and so on, forming a big diagonal matrix. May I know how can I do that?

for example : my Matrix will need to consist : M1₁₁M1₁₂M1₁₃,M1₂₁M1₂₂M1₂₃,M1₃₁M1₃₂(M1₃₃ +M2₁₁)...