I like to think of the History function as a kind of “third split.” It’s a nice function as a retroactive tool (where was that document I opened a few minutes ago), but it is also surprisingly powerful as a strategic tool, too. Just click on each document you wish to rapidly switch between in one split, and then use Cmd-[ and ] to scroll between them from that point on. But yes, if you actually need two other documents visible, then the suggestions using Notes and Scratchpad are best.