The config allows you to use 3 n!n, one for the dongle, one for the left half, one for the right half. Using a dongle significantly increases battery life. The dongle can be any device on the nRF52840 (or other supported ZMK controllers). More details on the slicemk page.
Before flashing this firmware, flash the setting reset firmware into all 3 controllers. Then flash this firmware on 3 devices. If the halves do not connect themselves, try pressing the reset buttons on the dongle and keyboards.