3-Way SLI/CF Performance Scaling
 

The first thing to take away is that 3-way graphics systems are absolutely not something anyone without a 30" display needs or should even want. There are so many instances where 3-way fails to improve on 2-way, and because of the high price barrier to entry and the diminishing returns on adding more hardware, 3-way is just not for everyone and has questionable value for those who choose to go that route as well.

Honestly, NVIDIA and AMD trade blows, but the one clear thing to note is that 3-way GTX 280 and 285 are just way overpriced. You don't get what you pay for and you can still get huge performance from the GTX 260 which offers consistently better value. On the AMD front, 3-way 4870 1GB pairing a 4870 X2 and single 4870 1GB is the way to go if you want 3-way. The 4850 can scale in some games that don't crush it with too much data and it offers better value in some situations.

http://anandtech.com/video/showdoc.aspx?i=3518&p=1