yeah all 3 monitors are going to be 27inch. same ones. 1920x1080
you'll be just fine, and if you want just using a single 780 will do you fine, just use adapters and you'll be set
edit;
I forgot to mention this a long time ago too, having two cards does not increase your amount of ports.
Only one card (with Crossfire, not sure about SLI, pretty sure it's the same), can have active ports