The mean of “但是 可是 不过” are almost same, but it is different to express your tone of speaking.
The tone of “但是” is the most serious, it almost emphasizes the content of certain things.
But the tone of “不过” is more indirectness than “但是”. “可是” almost emphasizes the contrast.
For example: The wind has stopped,”可是” the rain is still raining. Raining contrast stopped..