Not working on third monitor: graphics card + internal graphics

[csfive]

I've seen a lot of posts on issues with multiple monitors. I know what the problem is on mine (which might help others), but not how to fix it.

I'm using internal graphics for one monitor, attached via VGA (also tried HDMI). I'm using a graphics card for the other two. For my computer, in order to use the graphics card and internal graphics, you need to set the internal graphics as primary in the bios. This causes issues at startup (you have to log in without use of any monitor, the first time), but those issues go away when the adapter for the onboard graphics is disabled in device manager. Unfortunately, disabling that disables f.lux for that monitor (which does work if the Intel graphics adapter is enabled). The program can communicate fine with the graphics controller for the two monitors attached to the graphics card (NVIDIA adapter still enabled), but not the third. Honestly, I'm not really sure what is controlling the third - the NVIDIA control panel doesn't recognize it, but it's working (and also works with third party programs like DisplayFusion). Unfortunately I don't know how to fix this, so any advice is welcome. I changed my setup so that the monitor I use least is attached to the onboard graphics, so I can just turn it off if it's late and I don't want the bright light.