There is only 1 option left for you in this case, the USB to
hdmi option, but its not guranteed to work.
I use a HP with a desktop dock, so that gives me 4 physical
display ports, 2 hdmi, and 2 vga, but again maxes out at 2, just
like yours.
It is likely your graphics card does not support it, so you
might want to try the USB-HDMI option, but I personally have not
tried it myself.