If you have triple screens, you will want to use the 970's regardless simply because of vRAM. 680's, typically, have 2GB while the 970 has 4GB(3.5GB 'fast').
I assume you are running 1080p on the single monitor... I wouldn't use 3 680's there for sure. So much power use, potential noise, scaling issues, etc, its just not worth the hassle for 1080p. A 680 or two will be plenty. Also consider selling the 3 680's for a single card that uses 1/3 the power (thinking GTX 960 or a used 970).