1. Main screen. For the code editor, intense web browsing, etc.
2. Secondary screen. For debugging visuals (since I work on web stuff, it usually hosts a Chromium window; for a mobile dev, I imagine it would be an emulator/simulator), documentation referencing (with the code editor open on the main screen), etc.
3. Third screen. For all comms-related things: MS Teams/Outlook/Discord/etc.
I didn't mention terminal, because I prefer a quake-style sliding terminal. For a lot of devs, I imagine that having a terminal on their secondary screen permanently would work great as well.
P.S. Not that long ago, I realized that the physical positioning of monitors matters a lot (to me, at least) as well. I used to have 2 of them in landscape orientation side-by-side and one in portrait orientation to the side. It was fine, but didn't feel cohesive, and I definitely felt some unease. Finally got a tall mounting pole, and now I have the landscape oriented monitors one on top of each other instead of side-by-side (with the rest of the setup being the same). That was a noticeable improvement to me, as it felt like things finally clicked perfectly in my head.