In answering this question, I needed to multiply three matrices - which obviously involves writing them on the same line. I tried a couple of things, none of which worked, so my question is - how do you write collinear matrices?
It works the same way you did it earlier in the answer, when you had a matrix and vector: just write the markup for the matrices one after another.
They don't all have to be on the same line, but as with any other MathJax expression, you can't have a blank line between the starting and ending
I went ahead and fixed the markup in the answer for you, as an example.