using 3 monitors with macmini (2018)
From the Mac mini - Technical Specifications webpage:


  Video Support
  
  Support for the following combination of maximum concurrent display setups:
  
  
  Up to three displays:
  
  
  Two displays with 4096x2304 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096x2160 resolution at 60Hz connected via HDMI 2.0
  
  or
  
  
  Up to two displays:
  
  
  One display with 5120x2880 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096x2160 resolution at 60Hz connected via HDMI 2.0


So, yes, your model of Mac mini supports using three displays concurrently without using an external GPU.