Well fuck, take a look at this 3 screen PS3 GT5 gameplay.
http://www.youtube.com/watch?v=1ZB7kHPmuvEthe description says:
2nd test of GT5 with a triple screen setup. Hardware used is:
3x 60GB first edition ps3's
all hard-wired into a switch - as over wifi there was some slowness in framerates and even some locking up.
3x copies of GT5
Visionracer VR3 racing seat with custom triple screen monitor mount
3x Acer 3D LCD's
Logitech G27 racing wheel with pedals
Logitech X530 5.1 speakers with Sub
THREE PS3's needed for triple screen?

damn. well I guess we have our answer here... I didn't know you needed three different plugs, hmmmmm I gotta look more into it