GPU 3 is now in open beta testing. You can get the client from the link in the announcement page at The Folding Community Page.
So far, I'm unimpressed with GPU 3 as on the only card I've tested it on gives about 10% less PPD than GPU 2. Looking at some of the posts on the community forum it seems that only the latest 400 series cards give any points advantage. Probably 6-9 months before GPU 3 is in general release, I'm hoping The Pande Group to increase the points value before then.