The GTX970 will only SLi with another GTX970, it will NOT SLi with a GTX980. A GTX 970 will drive 3 monitors but you might need adaptors as the 970 has mostly displayport ports and only has a single DVI and a single HDMI port.
Personally, I have 2 main systems. One with a Radeon HD R9 290 4GB, the other with a GTX Titan 6GB. They can both drive my desk setup which consists a 4k centre screen (displayport) and a pair of 1080p screens flanking it (1off DVI and 1off HDMI).