Ive got an 890FXA-GD70 running 4x 5850s. They run hot... Ive got 2x 120mm fans supplying them air, under an open window. I changed the thermal paste to increase heat dissipation, but even after that they're still running hot. Temperatures are 76 - 83 - 89 - 84. The card with 76C is on the bottom, pulling cold air off the floor. The other cards do alright, but that one card in there pushing 90 worries me.
If you do buy that board, I would think 3x cards would do alright, so long as you leave the case side off and have a couple fans pushing air into it. Oh, and if you decide to use 4 gpus (dont know if that board's pci-e can handle it) make sure you buy a case that has enough slots
I didnt, and had to do a bit of modding.