OK, I did check out the BIOS/UEFI/Setup and all five fans can be configured individually. There are five options.
Control can be disabled, auto (the default), DC, or PWM. Wind-up time is set to 0. Minimum speed is 200 RPM. Profiles were Standard (the default), Silent, Turbo, and Manual. I set the three case fans to Silent, let's see what happens ... (Maybe the "auto" control doesn't work, I can try put that explicitely DC for the 3-pins and PWM for the 4-pins.) -- underground experts united https://dataswamp.org/~incal