Just one. Now let's read Toms about the i5 vs. i7 and driving 3 GPUs.
"Our tests demonstrate fairly little difference between a $225 LGA 1155 Core i5-2500K and a $1000 LGA 2011 Core i7-3960X, even when three-way graphics card configurations are involved. It turns out that memory bandwidth and PCIe throughput don't hold back the performance of existing Sandy Bridge-based machines. "

OK, so you have the 780 and want a little more punch. I'd read a few googles like:

http://www.tomshardware.com/answers/id-1790790/optimized-nvidia-control-panel-settings-gtx-780.html is a good read.

Then again, the 980 is one fine card.