Here's how to unlock those three hidden achievements.

1. Postman:
Keep swinging in front of the door to generate letters, you'll get an achievement.
This achievement can be unlocked at any time.

2. Madness:
Stay in your bedroom for about 5 minutes and you will get an achievement.
In the first run, this achievement needs to be unlocked on the second morning. After completing the game, it can be unlocked at any time.

3. ∇:
Input the following Morse code in the living room using the Z key or any other interactive key: "...._ ...._ ...._".
Every time you finish typing a set of "...._", you will hear a beep. Enter all three groups to unlock the achievement.
In the first run, you need to trigger the hidden dialogue on the TV before unlocking. In the second run, you can directly enter the Morse code in the living room to unlock the achievement.

The following link contains a sample audio of “...._”, you can refer to it for entering:
https://en.wikipedia.org/wiki/File:4_number_morse_code.ogg