Optiplex Three Monitor Debacle

I have a user on an Optiplex 3010.  The PC has a VGA and an HDMI onboard graphics connections.  It also has an add on graphics card with a DVI connection and a Display port connection.  For years, she has been running two monitors with them plugged into the add on graphics card and everything has worked fine.  There has been nothing plugged into the onboard connectors.  She recently asked for a third monitor.  I thought "easy peasy, I'll just plug a third monitor to the onboard VGA connector."  When I did that, only the third monitor works.  I checked in the bios and '...multiple monitors..." is checked.  If I shut down, unplug the VGA cable from the PC, and reboot, the original two monitors work again.  What am I missing here?