They are totally different meaning in Chinese sentence structure. In English , "but" and "however" are same meaning but different tone and grammar using on the sentence structure. Now we are talking about "可是","但是","不過", for example : 1) 雖然你這次失敗了，可是你得到一次好的人生經歷。 Something was happened, 可是this happened may bring you or you get or you lost after this happened. 2) 雖然你這次失敗了，但是你贏得一埸友誼。 Something was happened, 但是 you get something new. 3) 雖然你這次失敗了，不過希望在明天。 Something was happened, 不過you can think about that happened to be good or bad. ( like wish, hope, opportunity ) You can find out some sentences which is using with one of this three words, and then we can discuss here. Cheers!

汉语的书面语反对重复，遇到转折的场合，你可以试着使用这三个不同的词，变换使用，使你的文章更有文采，一般说，“但是”最书面化，转折的情绪最强烈，“不过”转折的情绪弱些。在很多情况下，是不用区分这三个词的。口语中，你如果说但是，听话的人会马上意识到你要改口反悔，你如果说“不过”或“可是”，可以减少对方的警惕性和敌对情绪。

这是三个不同的词，有不同的用途。如果你的汉语不好的话，我解释你也看不明白，我用英语没法给你解释。你记住一点，它们是不同的三个词。它们也许有部分交集，但它们是不同的。

They differ in ,many fields: first for meanings: 可是=但是 show a moderate negation, 不过 shows a slight negation. second for styles:可是and 不过 are usually used in spoken languages, 但是 can be used in both spoken and written languages.

basically they are not much difference. 但是 often occurs in formal written discourse. 可是 and 不過 occur more frequently in casual spoken discourse.