It looks to me like pci-e x8_3 is split into x16_2 and x16_4 with the NF200 splitter chip if you use either of those slots, effectively disabling the x8_3 slot.
It is possible to run both gfx cards on ports x16_2 and x16_4 and put the SAS card in x16_1. This will result in a lower pci-e lane count between your gfx cards and the cpu, but most reviews indicate that the every day effect is slim to none. So in your case this is what I'd try.
ROG Crosshair VIII Dark Hero
AMD Ryzen 7 5800X3D
32GB G.Skill Trident Z Royal
2TB WB Black SN850X
Soundblaster ZxR