afaik they added support lately.
There are some cards, that fglrx doesn't support atm.
Yours is one of them. Maybe you can fix it by adding the PCI ID to the source code, but no I don't know.
I just had a look and they are already there:
FGL_ASIC_ID(0x9501), // ATI Radeon HD 3870 (RV670 9501)
FGL_ASIC_ID(0x9505), // ATI Radeon HD 3850 (RV670 9505)
So I don#t know.