AMD 890GX chipset has one PCI Express 2.0 x16 lane that can be split-up into two x8 lanes if necessary. Translation: motherboards based on this chipset can have one or two x16 PCI Express slots, but they will work at x8 if two video cards are installed. The north bridge chip supports a total of six PCI Express x1 lanes, while the south bridge chip supports two more.
The ASRock 890GX Extreme4 comes with three PCI Express x16 slots. The first two work at x8 if both are used at the same time, while the third slot always work at x4 (using four x1 lanes from the north bridge chip). All slots support CrossFireX mode, as expected.
The motherboard comes also with one x1 PCI Express slot and three standard PCI Express slots.
This is the exact same configuration used on the 890GX Extreme3 model, but on this other model the third PCI Express x16 slot is painted white to remember us that it works at a lower speed (x4).
It is important to note that the 890GX Extreme4 supports three dual-slot video cards using a regular seven-slot case. With some motherboards that have a third PCI Express x16 slots, the third slot can only be used by dual-slot cards if an eight-slot case is used.