Does the hadith "Healing is in three" mean that healing is restricted to these three only, especially since there are other medicines mentioned by the Prophet, peace and blessings be upon him, such as black seed and Indian costus? And is the honey mentioned bee honey or black treacle?

Healing is not limited to the three mentioned, but rather they were mentioned as fundamental principles of treatment and its basis, due to their great benefit by customary practice. For Allah has not sent down a disease except that He has sent down a cure for it, and healing may be found in other medicines. The honey mentioned is bee honey.