Thanks for asking!
This would appear to be a hardware limitation in the system. This computer uses NVIDIA Optimus Technology to share resources between the Quadro K3100M and the Intel built-in graphics. This allows for extended battery life by utilizing the Intel graphics for light workloads but then switching to the NVIDIA Quadro K3100M when more performance is needed.
By linking the graphics controllers they also share limitations, in this case the Intel graphics is only capable of supporting three total displays including the built in display and two of the three external outputs, limiting the system to three total displays ( the internal display is considered to be always connected even when disabled in Windows ).
DisplayPort MST ( Multi-Stream Technology ) can be used to daisy chain monitors however I doubt it will work with this system due to age, additionally the graphics controller will still be limited to two external displays. Right now the only major benefit to DisplayPort MST is reducing the number of cables to the system, this is offset by configuration issues, poor hardware support, lack of documentation on supported systems and connectivity problems.
For which graphics card controls witch display, based on the Dell support forum article the Intel Controller is hardwired to the internal display and VGA output, while the NVIDIA card directly controls the DisplayPort and HDMI ports, however with shared resources there should be little to no performance difference between connected displays.
Please let me know if this helps, or if you have any additional questions.