yep, bystander is right. I've heard of this before as well, even some reviewers complained about it. Has something to do with different signaling/timings among the plethora of connection types used on those boards.
It might get fixed through drivers at some point, or not.. eyefinity isn't quite finished yet. *rolleyes*
So there's nothing actually wrong with your hardware as far as I know. Ideally you would need to use connections of the same type, but that means using 3 DP's and that isn't going to happen anytime soon. Even though the card supports DP1.2 and monitor daisychaining and DP MST hubs, neither are available.. (hubs or 1.2 capable monitors)
edit and if you google 'eyefinity tearing' you'll get tons of forum threads, some with weird solutions to fix it...