If you have an 1156 or 1155 socket CPU, you probably have an onCHIP GPU (iGPU) and won't be on the motherboard. But of course, some boards don't support them and may be the case. Just checking! Lately, you need "H" series boards and not P or Z series.
I go by the ports on the back of the motherboard, there's simply none I can plug anything video-related into. (MB is Asus P8P67 Deluxe)
Either way, running an AMD or Intel onchip GPU wouldn't be very pretty. It's fine for tasks, but once one screen gets decent load...stand by. Might wanna just look at your current GTX560 and count how many outputs you have. I'm looking at a GTX560Ti and it has two DVI + one HDMI and think it'd push 3 screens. If it didn't, I wouldn't need SLI to run a 3rd screen but just a second card. The last 3 x 23.6" set up I ran was about 8 months ago and was a GTX 460 and a GTX470...no SLI needed (nor recommended IMO).
If you have a GTX560, I see no harm in looking around for a deal on a 460 to drive the 2 side screens and keep the 560 on the main/middle screen since that's usually the one that is mostly used and want the horsepower. Heck, if you're "just" driving the 2 sidescreens with spreadsheets (like say, a day trader might), then even a very dated GTS250 will do just fine and not need but one PCIe power cord (Molex adapter would be fine here). That'll struggle a bit if you're still into FlightSimX or something like that.
My understanding is that only higher-level nVidia cards can go beyond two displays per card. There are three outputs on the card allright, but only two of them are functional at the same time.
Not doing SLI is another option I failed to consider.
The wole setup is this - main display (currently) for games/video/graphic work, supplemental display for random stuff (some background youtube or dictionary). The third display will be a TV, for HD video and some of the games. So I think hooking up main display and TV to GTX560 and buying something modest for supplemental display is the way to go, right?..