Its because there is no definitive answer.
The BIOS will have to have been updated from the factory. Given Ryzen 3000 came out a year ago, most boards sold now should have this update, but if the motherboard was produced a while ago and has been sitting on a store or warehouse shelf for a while, it may not have this update.
If an online retailer:
I would ask the retailer if the board ships with a 3rd gen compatible BIOS.
If in a physical retail store:
Look at the box and look for a "Ryzen 3000 Ready" sticker. If it has this, it will work out of the box. Otherwise, likely not unless the store updated the bios.
If you cannot verify if the board has the BIOS update or you just want to avoid any confusion, I would suggest any of MSI's...