What are you going to be doing that requires 3 HD6970 cards? You should be able to run Eyefinity on three displays with a single HD6970. According to AMD Website ( http://www.amd.com/us/products/desktop/graphics/amd-radeon-hd-6000/hd-6970/Pages/amd-radeon-hd-6970-overview.aspx#2
) each 6970 card could support upt to 4 displays in eyefinity and up to 6 displays using Displayport 1.2 connections.
Not saying you shouldn't do it. Just wondering what's the purpose.