TechPowerUp has benchmarked the AMD Radeon RX 6400 on PCIe 3.0 and has found out that the card experiences meaningful performance degradation. For instance, in Doom Eternal, the RX 6400's performance drops as much as 43% compared to the performance registered on PCIe 4.0.
https://www.notebookcheck.net/AMD-Radeon-RX-6400-suffers-a-double-digit-performance-decrease-on-PCIe-3-0.617891.0.html