Welcome!
I don’t know why you’d want to even mess with the existing circuits? I’d leave them be if practical.
If that’s the only power source than you might think about breaking up the shared 3 hots on one neutral and make two wires dedicated to a new circuits(a hot and a neutral) to the new cubicles and new breaker. You’d then have to QA all the receptacles in all the existing Cubicles to assure power. If you do feed a new circuit into this mix I believe you have to label it accordingly.
You stated 3 hots shared a neutral and one dictated circuit that makes 6 wires in use? Maybe you missed stating one circuit, use that and add a 12/2. But then again if your only adding from an existing whip your limited.
I’d run a new circuit if these cubes have there own whip and not mess with the existing circuit in the existing cubicles. You could run a 12/5 w/ 2 hots and two neutrals and for two breakers, for the new cubicles. One circuit each, beside people load up the cubes, JMO. They call it a 12/4 w/G, not a 12/5 which could be ordered!