it will depend on what graphics card you have. if the computer has integrated graphics or is a laptop then it is not possible
please post your system specs if you really want help with this
my system is crooshair v formula, amd phenom(tm)II x6 1100t processor water cooled,2x gtx550ti ,16gig ram,1tera bite hd,2x 19"monitor , 42" 3d tv as monitor
You need to use One DVI port on the card and the other the HDMI port on the same card or the two DVI ports on the card to drive two monitors.
The third monitor can be driven from any of the ports on the second 550Ti.