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.
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.