Why are teachers and doctors called three competitions in Japanese?

Because that's what Chinese calls it. Japan is from China.

1. Teacher, such as: Teacher Zhou (meaning the same as Teacher Zhou).

Mr. Lu Xun (Zhou Shuren)

2. honorifics for intellectuals and adults with certain status.

3. Someone who used to manage accounts or do paperwork. He works as a gentleman in a pawnshop.

4. In the past, it generally referred to people who read books, read faces, calculate divinations and watch Feng Shui.

5. Another name for a doctor: for example, "I am sick to see a doctor".