I´m using the "Arrange documents" button (top right) to view 3 documents at once. I get 2 smaller windows stacked on top of each other to the left, and 1 bigger to the right. Fine, but I want to be able to choose which document goes where. Say, document #3 ends up in one of the small windows but I want it in the big window. How do I change that?
This cannot be influenced. The order in which the documents were created or opened will always be used. Of course nothing stops you from re-arranging your workspace, of course, and moving windows where you want them.
Yes, I did a little try-and-error before and it seems to work like you said. First I consolidate all documents (1 window with three tabs), and I keep track of what document is currently active, then press "Arrange documents". Apparently, the active window (in consolidated mode) always ends up top left when switching to 3-up mode. So far I have not been able to determine what causes a document to end up in the big window.