@llek Welcome to Win-RAID forum.
HP have a proprietary BIOS that can only configured with the HP BCU tool.
revlaay’s tool is expecting an AMI BIOS and so cannot read/modifly HP BIOS correctly.
As the HP EliteDesk 800 G3 SFF has Kaby Lake support you might be able to get Coffee Lake to work but you will likely need a hardware programmer to flash the modified BIOS.
The ME Firmware needs to be downgraded from v11.8 to v11.7.1xxx (or v11.6) but there may be protection stopping you from flashing the Flash Descriptor via software.
You will need to add Coffee Lake CPU micrcocode (assuming there is room) OR replace existing microcode for CPUID 506E8 with another of exact same size (96,256 bytes) for your desired CPUID.
This is not a simple procedure and I would not attempt it without having a reliable hardware programmer so that you can recover in the case of bricking the PC.