Does that mean that you have a neutral connection, or not? If the three heating elements are connected in star, it’s not necessary to have a neutral connection, but you could have one. If there are three wires (brown, grey, black) plus the protective earth connection (green/yellow), you don’t have the star point connected to the neutral. If you have a blue wire as well, you do have a neutral connection.
What you might need to change will depend on the answer to that.