Well these are the 8.56 betas that I'm trying to install. As I said, they won't extract themselves into ATI/support folder like normal catalysts. They are downloaded in a zipped folder that includes the files that are extracted to ATI/support in normal catalysts install (like setup.exe etc.)
I also tried to use the modifier tool directly to this folder and it modified the .ini files correctly. But after installing them I still get the same error code for 3870 at device manager.
Anyway, thanks for your help so far.