in表示在某一范围内角落
The blooms are in the corner of the room.

on
1. 表示在某物的拐角上
The box is on the corner of the desk.
2. 有转角处的意思(外角)
Drop me off on the corner.

at表示在拐角周围；街口
Be careful when you are at the corner of the street.
My house is at the corner of the street.