Hi,
NoMachine server, and its corresponding license, is installed once on the machine which is running the desktop you want to access. In your case, a subscription license for Enterprise Desktop gives you access to the physical display on that host. If there are multi-monitors attached to that remote desktop (server-side), NoMachine will detect this in your remote desktop session and let you switch between the monitors. See here for more information:
https://www.nomachine.com/switching-the-view-between-multi-monitors-during-a-remote-desktop-session
If you prefer to start separate connections (so 3 in your example) to that desktop, they will all be connected to the same physical desktop, but as separate sessions each one will be able to view a different monitor.
I already tried ConnectPolicy autocreate=1,autoconnect=1,automigrate=0,desktop=0,dialog=0 on a Windows system, but still when I remote in, it disconnects the first connected session.
This key is for NoMachine virtual desktop sessions when connecting to NoMachine Terminal Server Products, so you should ignore this key. Have you already tried the evaluation ED or have you installed the free edition? Installing the Free Edition on the remote computer you want to access allows one connection. And so if you try to connect a second session, the first one will close.