You apply for de-registration if you wish to continue your studies at another university, if you wish to discontinue or interrupt your studies or after finishing your studies.

Yes, since you are enrolled at all three universities, you must accordingly also exmatriculate at all three universities.

You must apply for your exmatriculation by writing to the Student Administration Office at the end of the current semester. The signed application can be submitted by email or by regular mail. The processing time is usually 5-7 days. In case you need an immediate issuance of the de-registration certificate, it can be handed out during the office hours of the Student Administration.

The required exmatriculation forms can be found here:

Freie Universität: https://www.fu-berlin.de/studium/studienorganisation/immatrikulation/formulare/antrag_exmatrikulation.pdf

Humboldt Universität: https://www.hu-berlin.de/de/studium/bewerbung/formulare/Exma/view

Universität Potsdam: https://www.uni-potsdam.de/fileadmin01/projects/studium/docs/03_studium_konkret/08_formulare/studienbereich/a_exma.pdf

Exmatriculation generally takes effect at the end of the respective semester. The corresponding certificate will be processed and issued promptly after receipt of your application. If you want your exmatriculation to take effect earlier than the end of the semester, this must be justified. In this case, you must submit your student ID or campus card for the semester in question together with the application for exmatriculation. Please note that an exmatriculation with immediate effect is possible at the earliest on the date your exmatriculation application is received. Retroactive exmatriculation is not possible.

In the case of a change of university, exmatriculation with immediate effect is usually not needed if the semester start date of the other university is identical.