Can I connect two Full HD displays and one 4K display on 2018 Mac Mini?

A cursory glance of the Tech Specs would suggest so.

❝Video Support

Support for the following combination of maximum concurrent display setups:

• Up to three displays:
• Two displays with 4096-by-2304 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096-by-2160 resolution at 60Hz connected via HDMI 2.0
• or
• Up to two displays:
• One display with 5120-by-2880 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096-by-2160 resolution at 60Hz connected via HDMI 2.0

Thunderbolt 3 digital video output supports

• Native DisplayPort output over USB-C
• Thunderbolt 2, DVI, and VGA output supported using adapters (sold separately)

HDMI 2.0 display video output

• Support for one display with 4096-by-2160 resolution at 60Hz
• DVI output using HDMI to DVI Adapter (sold separately)

❞

https://www.apple.com/mac-mini/specs/