3 monitor setup (mac pro) with DVI splitter.. doesn't work?

I have an Early 2008 Mac Pro, I've added a third monitor to my setup which I was hoping to simply duplicate the main monitor's feed.

I bought a DVI-D splitter (http://www.amazon.co.uk/World-Data-xFFFD-Splitter-displays/dp/B004O2W8BE/ref=sr_1_1?ie=UTF8&qid=1422270043&sr=8-1&keywords=dvi+splitter), but when I connect it, my main monitor & third monitor just go black & the secondary screen then switches to become the main screen..

is there something else I need to be doing? I thought it would be a simple case of just connecting it all up...

G'day and welcome to the forums.

Upgraded the graphics card from standard? Have a read of this on suitable cards, ATI Radeon and nVidia, and DVI connectors:-


http://forums.macrumors.com/showthread.php?t=1709373