The manual only mentions TWO cards.
It supports Quad-Crossfire but that's likely two, dual-GPU cards.
I could be wrong but unless it's confirmed elsewhere I have to assume dual-card only.
*3xSLI/Crossfire tends to have more issues so I recommend staying away. In fact, if you don't have the cards yet I recommend NVidia since their multi-GPU solutions work better and NVidia supports the new G-Sync monitors which will be awesome.
I'm pretty certain AMD has still not addressed the issue of DX9 effectively running at half the frame rate.