How are the other two monitors connected? HDMI or display port or vga? Are they on the same RTX gpu? or on motherboard? or on another gpu card?
Sadly, Windows likes to change, or reorder, the default display (audio device) whenever it detects one of them going inactive. So unfortunately you might need an edid emulator on every monitor, or at least on the ones that you want to let sleep/power off.
The workstation's two monitors are both connected via DisplayPort outputs on the same RTX 3070 GPU as I'm using for the HDMI connection to my AV amp. That's the only GPU on this machine.