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.
I just dislike being told unless I follow these fads I'm a subpar developer. I don't own a mechanical keyboard or a three monitor setup. I don't own a 4K monitor (I suppose I eventually will, when they become the norm). When Apple came up with retina displays, I didn't feel I had magically become unable to write code because my display at the time was 1440x900.
It's weird to me to specify the number of monitors given how they come in a vast range of shapes and sizes.
For example, my dream setup used to be a single 55" curved 8K monitor. That's the rough equivalent of a 2x2 grid of 27" 4K monitors (I currently have two 27" 4K side by side in landscape @ 1:1 scaling).
The only problem with my so-called dream set up though is I don't think my computer glasses, which are sharp only for surfaces between a range of 21" to 27" would allow me to see everything sharp from corner to corner on that monitor, which sucks.
Also then when one of my disastrous kids destroys it I don't feel bad.
on edit: fixed typo
I found it to be less distracting and the two screens are more physically manageable, and easier to replicate if i change setting (cheaper too!). The only thing I will change is whether I'm in landscape/landscape or landscape/portrait. I can never make up my mind about what I prefer.
Email hides behind the web browser and slack behind the output.