So this has been bugging me for a while now. I've been looking for a X399 motherboard with three PCIe 3.0 slots that function at full x16 speeds. However, only the Asrock Taichi X399M has such a layout, all the others are x16/x16/x8/x8. also need one PCIe x1 2.0 slot which the Taichi X399M doesn't have by virtue of being a mATX board and thus has to skimp on slots.
So yeah. Why aren't there any boards out there that offer only three slots albeit with all three functioning at x16 speeds aside from that one Asrock offering? I think there are valid scenarios where having all three PCIe 3 slots at 16x is useful (ie first two slots used to drive GPUs in CFX mode, third slot running a separate, standalone GPU driving a 4K touch panel, or third slot running four NVMe drives in a 4x4 adapter), plus the one PCIe 2 x1 slot for a SoundBlaster.