When my buddy Austin Kleon posted his notebook turducken I realized that I have a three-part system too, though a somewhat different one:
I use a small Moleskine planner for my calendar and tasks, a Leuchtturm notebook for ideas and drafts, and index cards for reading notes (including notes I make for the books I teach). It would of course be possible to put all of this in one notebook, but such simplification would come at too high a price. For instance, with my current system I have a place where I keep track of my events and tasks for an entire year, but if I did that in the notebook where I also write drafts, then when that notebook was full I’d have to make a new calendar no matter what point in the year I’m at — and then lose ready access to the old calendar. Similarly, when I have reading notes on index cards, I can spread them out and look at them as I’m writing drafts, which is much better than having to turn back and forth to multiple pages in multiple notebooks.