They all mean "to sign up", but we normally use them in different situations:
Cadastrar-se: it means to sign up to something.
Example: Eu me cadastrei em uma newsletter de tecnologia.
Registrar-se - it means to sing up, but it can also mean mean to register yourself.
Example: eu me registrei nesse website.
inscrever-se - it means "to subscribe" or "to enroll".
Example: eu me inscrevi em um curso. Eu me inscrevi em um canal do Youtube.
In your case, it would be ideal to use "Registre-se" or "Cadastra-se".