Because the monitor remains on even after you reduced the timeout to 3 minutes, the next step I suggest is checking whether something is actively requesting that the display stay awake.
- Open Command Prompt as Administrator and run:
powercfg /requests - If something appears under DISPLAY, temporarily close that application or disconnect the associated device, then test again.
If you'd like, please share the command output here and we can use it to narrow down the cause.