If you are going to be using 3 1440p monitors in surround 3Gb may not be enough. http://complaintsincorporated.com/2013/12/15/1440p-surround-tri-sli-testing-part-1/
Even 2 6gb 780's might struggle at that resolution.
I don't believe that, 6GB VRAM can at least run 5 monitors if one monitor takes one GB.
It went above 3 gb of vram, not 4, meaning FPS would drop, so there is an issue with VRAM? 6gbs wouldn't do the job either?
Two 290Xs can get teh job done on par with the gtx 780tis but they have 4GB VRAM so you'll have 1GB of leeway if each monitor takes 1GB or VRAM
It depends on the game. Even @ 1080p Wachdogs(as poorly optimized as it is) can use 3.7Gb of vram(see this on my rig). Even BF4 on a single 1440p monitor will use ~1.5-2Gb of vram(depending on msaa settings), so if you are using 3 1440p monitors on surround you are tripling the amount of needed vram. http://www.guru3d.com/articles_pages/battlefield_4_vga_graphics_performance_benchmark,11.html
New games are requiring more vram. Even Wolfenstein will get close to 3Gb of vram usage @ 1080p. 3gb would be fine for a single 1440p monitor but I would hesitate to use that much with 3 running at either 7680x1440 or 2560x4320.