Associative property of matrix addition is also true. That means, the grouping of matrices doesn't matter when adding three matrices.
For example, if there are three matrices A, B and C
A = [1 2 3], B = [4 5 6] and C = [7 8 9]
Then, A + (B + C) = (A + B) + C
It does not matter in which order you group the matrices, the result will be same in all cases