Used this AMD official reply concerning PCIe 4.0 lanes but also mentions about 3rd Generation AMD Processors: https://community.amd.com/community/gaming/blog/2019/05/27/the-industry-leading-amd-am4-x570-chipset
Best to open a Support Ticket with your manufacturer's Motherboard and asked them if they will create a new BIOS version to support 3rd Generation processors.
If not, you will probably find out once the actual 3rd generation processors goes public for sale. At which time is when any new BIOS will come out to support it if it will by the motherboard's manufacturer.
