That's because their are none. They have been no new AMD chipsets in a few years, so PCI-e has never been added to the AMD's. In reality, no card can max a 2.0 slot anyways, so 3.0 isn't really a concern.
EDIT: I guess there are one or two
http://www.asus.com/ca-en/Motherboards/SABERTOOTH_990FXGEN3_R20/
being one