3rd card offers diminishing returns: http://www.maximumpc.com/geforce-gtx-980-ti-3-way-sli-overkilling-it/
Some great data there including minimum/average frames and % scaled from 2 to 3 cards.
2 should suffice - even for 4k gaming , you cant get more than 60hz monitors. i get 40 - 60 fps depending on title & MSAA settings @ 4k with a single 980 ti.