I was able to use zonescreen to simulate a 4th monitor.
I have an ATI 9550 with only one monitor connected to it.
What I did is I went into the ati catalyst software and forced it to detect a TV connected to the second port of the ATI 9550. I only have one real monitor connected to that card.
then I was able to enable that monitor in display/control panel. And then I used zonescreen, with the laptop as a client.
But trying to use it with zonescreen resulting in zonescreen, then FSX crashing.
So I geuss it's either the maxivista software or an used monitor from ebay. The difference in price is negligible if you are willing to go with a cheap LCD or CRT.