u have 3 monitors and not know properly what to buy? if u have money then buy the high end products! they will be able to better handle ur games any any case and settings
I got an Asus 680 gtx 4 gb running three older but good Samsungs at 5320x1050 plus an Acer 23" touch screen, got very good quality flightsimming going. So, a single but high memory card can do a lot. But again depends on what your gaming will be.
depending on the game a 3gig or 4 gig can do trippel monitor setup. as i stated, depends on the game. you dont need a titan-z thats just totaly bull TBH. you can get a normal titan ( the first titan non black edittion ) witch have "6 gigs" of memory. ( ask arne saknussen ). the first titan is between the 780 and 780TI when it comes to performance. but all in all it depends on how mutch you want to spend realy.
A pair of GTX 780's would do the trick, or CF R9-290's...
Personally I would go for the AMD option now as they have 4gb of Vram which is helpful at high screen resolutions. I doubt a single GTX 780Ti would produce decent framerates, however I suppose it depends on the game. BF4 and Metro Last Light may be problematic but some of the Unreal 3 and source based games should be fine.