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.
Choose Apple menu > System Preferences, then click Displays. If your Mac doesn’t recognize a display you’ve just connected, hold down the Option key to make the Detect Displays button appear, then click that button.