hello,
i think it depends on the video driver and i remember on my old pc that ati catalyst was capable of using multiple monitors as one big screen, so in that case full screen fills all monitors, but on my actual nvidia based pc, that's not possible, because full screen expands only on the monitor where the application is launched.
In my opinion you have 2 alternatives:
launch your application in normal mode(not full screen) and set the width equal to the sum of your displays width resolution and the height equal to the display with the lower resolution(it's better to have all displays with the same height).
Alternatively you could split your application in 3 standalone applications(each one on one display) communicating over the network module.
Sal