Try connecting them all on one card, and see if it works, nothing to lose.
I have never had someone able to try that, but the hardware configuration is sounds for 3 outpus (2 driven by TMDS and 1 driven by RAMDACs), but it could work. However in the initial documentation they only mention combinations with Display-Port involved in some way, not a card that has no DP connector. but that it's not mentioned in their documentation, doesn't mean it won't work (ala passive DP->VGA). Since it doesn't cost you anything, try it and see if you can fiddle with the control centre and make it work.
You would however connect all three to one card, and then use the second to simply Xfire the extra horsepower.
Dual cards will work for something like Flight Simulator X , but it's not using Eyefinity which runs off one card not two (unless there's been a recent driver update to drive 2 cards with separate sets of monitors).