I bought an auros elite b450 it's compatible with the new Ryzen cpu's but needs a bios update to work, check the motherboard manufacturer website under the bios section to see it there is an update available.
Both of those motherboards will need an older AMD CPU installed to update the BIOS before you can use the 3xxx CPU.
You can probably get a local PC shop to update the BIOS for you OR you can get a loaner CPU from AMD to update the BIOS:
Scroll down and read the section titled:
Short Term Processor Loan Boot Kit https://www.amd.com/en/support/kb/faq/pa-100
I have the ROG Strix B450-F Gaming and I am very happy with it and I am fully confident it will handle anything up to the 3700x without any issues once ASUS gets the bugs worked out of the new BIOS updates.
Also all the new 3xxx BIOS versions from every manufacturer has bugs at this point so I'd wait a month or so and see if they have a stable BIOS released for whichever board you like before buying.