Using 3 monitors on a 2 monitor card?

So here's the deal. I have a rig with a 6950 that has for its entire life been a 2-monitor rig. However, after a move I've added a 3rd monitor to the mix. I plugged all 3 monitors in to my 6950 (2 DVI, 1 HDMI), but the HDMI wouldn't function. I don't understand the specifics, but the HDMI port shares some components with one of the DVI ports. Anyway, I plugged it into my motherboard HDMI port and it's now working.

So here's the question. What components are powering display on this 3rd monitor? Is it the onboard graphics on my 2500k? Or is it still my GPU that is just pushing its information through a different hardware lane? If I'm playing a game in a windowed environment and I move it over to this 3rd monitor is it going to hitch? That wouldn't really make sense to me though. I'd just like to understand this a bit more technically is all.

Reelevant parts for reference:

its the integrated. i have the same kind of setup (960+integrated on g3258). works well but dont game on one while using the other to render, its poo. lol.

Would I have problems gaming on one while watching Netflix or a movie on the other?

Those cards can run up to 6 monitors though the 2 display ports.

This is true, but I'm not really interested in buying a hub and adapters, as none of my monitors have display-port inputs. Just at the moment.