2018 Mac Mini 4k Scaling issue
If you had a larger monitor, the problem would go away. I also have a 2018 Mac Mini. I am using a 49" 4K TV as a monitor. There's no scaling going on, and it looks great. The TV is a TCL brand. Maybe the 49" size is a bit extreme -- I guess the 43" model might be better for most people. But, text is large enough without any scaling, and typing is responsive. The display is 3840x2160, and the Mac thinks it is driving a 36-inch display. 

I'm driving the TV using HDMI. The MacMini only has one HDMI port, so if you want three of them, you'll need some adapters. 

Figuring out where to put three 40-something inch monitors where you can see them all is another problem, I guess.