Which monitor is used to show the booting screen?
The Mac mini models, that support up to 3 displays, normally use one of the USB-C ports for the display with the initial Apple logo. The HDMI port would only be used for the initial Apple logo display when there are no USB-C port connected displays. I can only assume that if three displays were connected, then the same USB-C port connected display would always be the initial Apple logo display. In other words, you not going to know which of the two USB-C port connected displays will be chosen for the initial Apple logo until you turn on the Mac. And, that this choice would be consistent.