You are releasing buttons too early or too late.
If your Power button is malfunctioning or if your phone is currently connected to a PC and you prefer a software approach, you can use ADB (Android Debug Bridge). This method requires that your device is currently running the operating system (Android) and that is enabled in Developer Options. How to Open Fastboot Mode on XIAOMI Poco F3
Your Poco F3 is now in Fastboot Mode.