It is possible, just that Windows Display Control Panel/Settings user interface does not support it.
This applies to spacedesk and all other displays.
We are considering to add this option but we are currently very busy with higher prioritized features and bug fixes.
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 - it will only work if your primary machine is Windows 10 and has latest graphics adapter with WDDM 2.0-above model (you can check it by running dxdiag.exe)
- go to spacedesk server window and click "Clear All Display Settings" on menu
- connect up to 3 spacedesk viewer and make sure that all viewers are in extension mode
- press "Windows key + P" combination, then select "Duplicate"