Hi,
You have a hardware event, see this site (Link) for troubleshooting a 3:4 BIOS error code. This error code is located in the first group at the above site as your PC was manufactured in 2015.
You replaced the power supply, swapped RAM, and tried different keyboard and mice.
You might have to try a different, known good, stick of RAM in both sockets, disconnect SATA storage devices (HDD and DVD), remove the M.2 WIFI adapter, disconnect the USB card reader.
The last item would be the motherboard or the CPU. CPU failure is very unlikely but can happen.
Regards