Matrix multiplication is associative - it doesn't make a difference "when" you multiply two matricies together (although you have to keep the order straight since it's not commutative.).
If A, B, and C are any matricies such that the dimensions make their product meaningful, then (AB)C = A(BC).
ABC can be calculated by first multiplying AB, then mulitplying the result on the right by C, or
ABC can be calculated by first multiplying BC, then mulitplying the result on the left by A,
and the result will be the same either way.
i.e. (AB)C = A(BC).
("God does not care about our mathematical difficulties. He integrates empirically." - Einstein. That's a wonderful quote!)