4.0 is backwards and forwards compatible with 3.0. If you insert a 3.0 card it should automatically revert to 3.0 mode. Based on the Aurora R12 service manual, there is not an option to set the operation of the PCIe slots from 4.0 mode to 3.0 mode. Service manual located here   What type of card are you trying to install?
How do I change the bus from PCIe 4.0 to PCIe 3.0? You lock down your BIOS that we cannot change advanced settings. Please let me know.
Aurora R12, PCIe 4.0 to 3.0