Word is out on the streets that the upcoming high-end AMD RD790 chipset may have support for three-card CrossFire.
Currently, using two cards in CrossFire gives you about 1.8 times the performance according to AMD and with three cards, you should be getting 2.6 times the performance according to information on HKEPC.
With HyperTransport 3.0 and PCI Express 2.0 there should be enough bandwidth for more advanced CrossFire options and it will be interesting to see what AMD is working on here.
HKEPC is also talking about a potentially super low TDP for the RD790, as it's meant to draw a mere 3W at idle and 10W at full load. Compare this to the Intel X38 chipset that draws 14.4W at idle and up towards 35W at full load.
The AMD RD790 chipset is expected to be ready in late September.