I just got the Hyper M.2 X16 Gen 4 card working in an ASUS X570 ROG Crosshair VIII Dark Hero motherboard with four Western Digital SN850 M.2 drives. I have two RAID 1 arrays and I am booting off of one of them. The Hyper M.2 card is installed in PCIe slot 1 and the slot is bifurcated x4x4x4x4. Unless I am mistaken, slot 3 on your motherboard is on the chipset and likely only supports 4 PCIe lanes. As a result, my guess is that you will see at most, 1 M.2 drive on the Hyper M.2 board installed in slot 3. You may need to move it to slots 1 or 2 if you want more drives installed in the expansion card. Check out this document:
https://www.asus.com/us/support/FAQ/1037507/.