AMD have announced that they will be offering Zen 3 support for older AMD 400 motherboards. This backtracks their previous statement that there would be no such backwards compatibility support.
The main reason they did not offer support initially was due to "serious constraints in SPI ROM capacities in most of the AMD 400 Series motherboards". However they have found a novel solution:
AMD made a commitment to support AM4 socket until 2020 and it looks like the newest batch of Ryzen 3 CPUs will be supported for even longer than expected.