X570 and B550 are the motherboards to pair with your new Ryzen 3000 processor, but if you want true value there will be the unannounced A520. And that chipset has been spotted at ASUS.
A total of motherboards have been spotted based upon this chipset and appear on their support page, a common source of leaks as they list models before launch, and are as follows:
From the looks of things, these are all mATX form factor motherboards and fall in the Prime and TUF Gaming lines, there is no mention of a Strix line. With the launch of the new Ryzen 3 3100 for US $99, these motherboards along with one of these processors will undoubtedly offer good value. AMD has not made any A520 chipset announcement just yet.
Asus is listing 5 motherboards based on affordable A520 chipset