I have a Lenovo T440 laptop on 12.04 with two external monitors attached (running through one VGA port that goes to this splitter). Right now Ubuntu is counting the two external monitors as one (they mirror each other). It seems like 3 monitors is a bit tricky. For now, how do I get my laptop screen to mirror one (but not the other) monitor. That way my laptop and one external monitor will show one picture and the other external monitors will show the other. Thanks.
The specifications on the link which you provided for your splitter says:
"provides an inexpensive solution for displaying a mirror image on 2 monitors at the same time"
What you want is not possible with this equipment.
I believe one is connected to HDMi. I don't know your laptop specs but without the necessary drivers the HDMi would either not work or act like a VGA out. So the easiest solution would be to install latest closed-source drivers for your GPU.
I am pretty sure there could be open-sourced and code-based solutions too but I have no idea about them...