You can overclock your GTX 970 to match the power of a GTX 980. If you get two GTX 970's and overclock them to match the power of a GTX 980 sli config, you will easily run mot games at about 50 FPS. 4k has four times as many pixels as 1080p, so with triple 1080p monitors your GPUs will need 0.75 times as much power as 4k, and that's well over 60 FPS (the most common monitor refresh rate) in any game except very graphically demanding games that don't even support SLI such as Crysis. You can also throw in MFAA for no-FPS-cost 2x MSAA. But don't worry about the numbers, your system will kick arse any way you put it.
Here's a link to the overclocking proof:http://techgage.com/article/taking-it-to-the-limit-overclocking-nvidias-geforce-gtx-970-980/
Here's a link to Techpowerup's review of GTX 980 dual SLI:http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_980_SLI/