to be honest,"可是“但是”(formal)，their diffrence are not obvious in china.you can use both when writing some works.especially for composition , ,"可是“但是”are ok.you don't need to distinguish them clearly.to "不过"(informal),we usually use it in spoken chinese.you can hardly find it in the cultural works.

All of the words are the same ! 可是,不过,usually use in people's native wor, not read.but 但是 use in book often.

your question is very well there is no difference in our daily communication, as a foreigner you know the mean is but that all enough more chinese questions i can help you my major is china email [email protected]

for myself the three words (可是, 但是, 不过) don't have too much difference except the feeling. 但/但是 is more common and 可是 is a little softer and more polite, i think. 不过 means "but" but i feel it's perhaps closer to "except for".

they are grammatically equal, the difference lays in the application, as the friends above said. my suggestion is to read more, that helps you to form your own style to speak the language, like Chinese do.