No, they don't support 3 PCI-E 2.0 x16 slots. See
here.
They only support 2 PCI-E 2.0 x16 slots. Another PCI-E Grapics port can be added using the left over PCI-E lanes provided by the chipset. The x38/X48 chipset provides 38 PCI-E lanes. 32 of those are used up by the 2 x16 graphics ports, that leaves just 6 for the rest of the components. Another lane is used up by the audio card on this board, that leaves 5. Another is used up by the IDE controller, that leave 4. Chances are ASUS also using another lane for the Mavell Gigabit LAN controller, that leaves 3. So with this board, best case, is that you get a third PCI-E Graphics slot that is physically x16, but electrically only x1(because you can only have x1, x4, x8, or x16). Not a good thing, and performance would be terrible with any graphics card plugged into that port.
They also could have split the two graphics ports up and added a third, so that when the third card is plugged in, two of the slots share the x16 and it gives you triple CrossfireX at x16/x8/x8, however then people would complain that it does give true x16/x16 CrossfireX and it make the board design a lot more complicated, which translates to even higher prices on a board that is already extremely highly prices.