Are you using the Rampage III Black Edition?
Check your motherboard manual for the PCB diagram which indicates which slots are 16x. The Raid card cannot be seated in the same PCIe bus channel as the Graphics Cards otherwise it will default the speed of at least one of your cards to 8x (sharing the bandwidth).
You can seat that RAID card in any PCIe slot 1x all the way to 16x (no problem). The card if seated in an x16 slot will still only function at 1x speed. Although, a 1x RAID card will give you much worse RAID performance than the onboard Intel SATA II Controller. I would not even recommend running a 1x RAID card. You should go with 2x or 4x in order to achieve solid Read/Write speeds.
If you still want to use that 1x card mount it in a lower PCIe slot. Boot and check your Bus Speeds. If still at 8x on one of the graphics cards move it to another PCIe slot and try again.
Motherboard: ASUS ROG Rampage V Edition 10
CPU: Intel 6950x 4.2GHz
GPU: 2x Nvidia ASUS RTX 3090 with EK Water Blocks
Memory: Corsair Dominator Platinum 64GB (4x8GB) 3,600 MHz
M.2 SSD: Samsung 960 Pro NVMEI
Sound: Creative XFI Fatal1ty Champion Series
PSU: Corsair AX1200i