It says that you can do it. But the thing is when you are in crossfire mode you know that you are only running the cards x8. When you install the 3rd card #1 card is at x8 and 2 other cards are at x4. Read this it is from their site.
1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode.
1 x PCI Express x16 slot, running at x4 (PCIEX4)
* The PCIEX4 slot shares bandwidth with the PCIEX8 and PCIEX16 slots. When the PCIEX4 slot is populated, the PCIEX16 slot will operate at up to x8 mode and the PCIEX8 will operate at up to x4 mode.
* When installing a x8 or above card in the PCIEX4 slot, make sure to set PCIE Slot Configuration (CPU) in BIOS Setup to x4. (Refer to Chapter 2, "BIOS Setup," "Peripherals," for more information.)
(The PCIEX16, PCIEX8 and PCIEX4 slots conform to PCI Express 3.0 standard.)