With the new series of Nvidia cards, you are not required to use SLI. You just need to connected all 3 to the card, using adapters as needed. However, a single 670 may struggle for performance. It's performance will drop to less than 40% of your current performance.http://www.tomshardware.com/reviews/geforce-gtx-680-sli-overclock-surround,3162-6.html
You'll see that at 1080p, it averages at 70 FPS in Dx11, and drops to 27 FPS at 5760x1080 (3 monitor resolution).