What do you mean by putting three nails under the bed?

1, you didn't notice the scattered nails on the bed board when you made the bed;

2. The nail fixing the bed board moved and fell off by itself, just like there was paper;

3. Is there anyone who likes to play pranks in the dormitory? If there is, I don't need to say it;

4. Are there any superstitious students in the dormitory? If so, it may be some superstitious activity.

However, for whatever reason, it's no use thinking too much as long as you don't hurt yourself.