If your primary machine is Windows 10 then it is possible to mirror your desktop screen on your 3 iPhone device via spacedesk iOS viewer app.
It's just that Windows Display Control Panel/Settings user interface does not support it.
This applies to spacedesk and all other displays.
We can only suggest the following:
1. You can use any third-party software that can mirror the main screen to all monitors connected (e.g. Actual Multiple Monitors)
2. You can try this trick this will only work if your primary machine is a Windows 10 and has latest graphics adapter with WDDM 2.0-above model (you can check it by running dxdiag.exe)
- Connect all your spacedesk viewer apps (on your 3 iPhones) and make sure they are all in extension mode first.
- Then click spacedesk tray icon to show the spacedesk Server window, then click the menu button and select "Clear All Display Settings".
- Press "Windows key + P" combination, then select "Duplicate".