You can also do triple crossfire with one dual 6870 card and a single 6870 card. Problem is that the dual 6870 cards all only have 1GB of RAM per GPU so even though those cards are about as fast as the Radeon 7970 in some games at some settings they fail to perform well when they run out of RAM. Unfortunately they can run out of RAM long before they run out of GPU horsepower so they really can't be used in situations that would have made good use of all their speed.
Triple crossfire is shown to reduce micro-stuttering tremendously so it has it's uses in low quality settings where you want high frame rates and smooth gameplay cheaper than the more high end dual GPU setups offer. Three 6870 GPUs can outperform the high end dual GPU solutions while using between slightly less and significantly less power.
However, there are other problems besides the limitation to medium resolutions and settings like 1080p and lower on the triple and quad crossfire 6870s. One large problem is compatibility in games because some games don't work well with them if at all.
Have a look at this article where Tom's reviewed dual 6870 cards crossfired with a single 6870 for triple crossfire and crossfired two dual 6870s for quad crossfire. Tom's did a pretty thorough look into these cards and the problems associated with multi-GPU set ups here:http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995-15.html