There is simple answer. For 3 monitor you kind need more memory than 2GB.
Then 760 or 770? Well it depend lot on budget. 760 is in SLI really fast. Faster than one 780 or titan.
You can OC 760 to mach 770 performance. But if you OC 770 it is faster. How much is other story.
Not much. But here is good deal GTX 770 4GB card. http://pcpartpicker.com/part/zotac-video-card-zt7030410p
and 760 4GB card. http://pcpartpicker.com/part/zotac-video-card-zt7040610p
If go SLI then you need good psu. (big enough)
And yor cpu is not maybe best but you can upgrade it to K version or Xeon 1230V2 if you do not want to OC.
This is LGA 1155 build?
Then other option is get R9 280x or R9 290. But you want 3D so it is not a option for you.
But they work really well with eyeinfinity with 3 monitors.