Looks like both Asus and Gigabyte jumped the gun on B450 Zen 3 bios.
AMD Ryzen 5000 Zen 3 Desktop CPUs Reportedly Running on A320 & X370 Motherboards, B450 Support Already Added By ASUS & Gigabyte
AMD's Ryzen 5000 Zen 3 Desktop CPUs have been spotted running on A320 & X370 motherboards while B450 BETA BIOS support is already added.
wccftech.com