here's a little guide for you:
A320 = locked motherboard, don't get this, all Ryzen are OCable, so this is a really bad motherboard for them
B350 = offers everything that Ryzen needs, except Nvidia SLI certification
X370 = B350 + some extra feature + SLI certification
for me? i rather stick with B350 + R7 unless u're planning on doing SLI later on
what's your other spec anyway? and what's the purpose of the rig? i don't think slashing the price on the CPU in exchange for an expensive mobo is worth it