in order to use 3 screens on a single gpu, you will need at least a GTX 600 series card, or a HD 5000 series or higher(using a active display port>dvi for the 3rd monitor, unless you bought a sapphire flex varient which doesnt need it) if you use 2 gpus or more, then the above isnt necessary(but can cause crossfire/sli problems)
I cant really give a serious recommendation due to not knowing about how intensive a flight simulator can be first hand. but If i were going to run 3 screens, it would be 7950 or better(670, 680,7950 ghz, 7970, 7970ghz). I personally would not like to run 3 screens using a 660ti if its recommending gpus past the 300$ mark.
This is for a friend, and i haven't even ran the program myself. But i guess i will have to increase the budget a bit as the monitors and video card will take a good amount of the budget. Trying to see where i can save money.
I will give the program a try on my 2d surround setup using a single gtx 670 and see how it works out.