So I've upgraded to triple monitors & am having FPS issues. I can get 30-50 FPS. I turned the resolution down to 4080x768 & that helped. The goal is to run a steady 60 FPS. My specs are:
GeForce GTX 1050 2GB
8 GB RAM
The more monitors you add, of course the overall performance will decrease, simply your GPU has more to drive. The only way you'll get the best out of the gaming monitor (as I call it, I too have 3 screens) is to run the game in fullscreen or use a lower resolution on all your monitors, but of course that will sacrifice your viewing pleasure. Or, upgrade your GPU again to something more powerful.