Quote from the EFRA's site:
"All the currently old 2-chamber mufflers that are valid for 2005 in Off-Road will be removed from the list and the handbook 2006.
A proposal will be made by the EFRA executive end of this year for the AGM. So from 2006, if the AGM accept the proposal, there will only be 3-chamber mufflers on the official homologation list."
They definately exist, but I've searched a bit and haven't found any sites that mention 3 chamber in any pipe specifically. I would assume a 3 chamber pipe would probaby give better fuel economy with the trade off of less power, due to the increased back pressure. A 3 chamber would be quieter as well. I wouldnt worry about it though, just get a normal pipe that suits you needs, wether it be low end, mid, or high end power. Novarossi pipes are generally regarded as being some of the best you can get. If you want the most durable pipe out there the Jammin pipes are really tough and hard coated.