I had 2 external monitors IN ADDITION to my built in laptop monitor working perfectly on Windows 10 without any 3rd party software (or hardware), up until I updated my graphics drivers. After that I found I could only have 2 displays working at one time.
After a bit of searching (and found nothing), I tinkered with some settings until I found out how to ENABLE ALL 3 MONITORS to work again.
Just to clarify, I have Windows 10 on an ASUS R501VM laptop with HDMI & VGA output.
With both monitors plugged in, Windows display settings & Intel HD Graphics could only detect 1 external monitor at a time.
- Open "Control Panel"
- Select "Devices and Printers"
- You should see both external monitors being recognized.
- Right click the monitor that is currently not working and select "Display settings"
- From there you should be able to tick a box that says "Enable this display" or something similar.
- Click [Apply] and you should now have all 3 monitors working!
I hope this helps.