Mac Mini 2018 - 3 monitors
You can’t use three Thunderbolt 3 ports to drive three displays.   You can accomplish this by connecting two Thunderbolt 3 to HDMI adapters/cables and one HDMI to HDMI/VGA/DVI adapter via the HDMI port.

A couple of things to consider when connecting monitors:


Use native signals rather than convert.  Thunderbolt 3 is also DisplayPort, so it’s better to go from TB3 to mDP/DP
If you must convert video signals, use an active adapter instead as it actually recreate the native video signal