With macOS 10.14.6+, Final Cut Pro X 10.4.6+ automatically uses the GPU in the Breakaway Box, Breakaway Puck, or DuoModo eGPU Module is used to accelerate Final Cut Pro X.
Connect one 4k monitor to the DP port.
Connect one 4k monitor to HDMI port.
Connect two 4k monitors a Sonnet Thunderbolt 3 to Dual DP or Dual HDMI adapter connected to the Thunderbolt 3 port.