但是 = but 可是 = though 不过 = however they are all common use. just think of English how would you use them.

firstly， 但是has the strongest feeling. then, generally speaking they're the same , only some slight differences-in some occasion----- 1.但是 the two things before it and after it weight samely. 2.可是 thought don't want to accept the thing before but have to accept; then emphasis your idea or reason why you don't want to accept. 3.不过 the thing before it is generally agreed, the thing after it is special. eg: 1. 大家都说她很勤劳，但是我不喜欢她。（only the statement of my idea） 2.大家都说她很勤劳，可是我不喜欢她。(emphesis that I'm the one that don't like her) 3.大家都说她很勤劳，不过我不喜欢她。(I'm didferent from others.) not so clear, don't know if it helps.

good question. all of the three phrases go to the effect "but". and 但是 is the strongest and is the only one that can be used in very formal occasions like diplomacy, like the chinese big shots would always say, 中国让香港继续它的资本主义制度，但是前提是“一个中国”的原则。if we put Keshi or buguo, that'd sound very casual and improper. 但是can be used everywhere. 不过is shanghaiese, too :-)

“但是”is a bit formal and has the strongest sense from the aspect of tone. “不过”is used more frequently in speaking, weak in tone, and sounds polite. “可是”has a tone like “但是". It is exchangeable with it.

It's my own opinion and short answer... Maybe coz the long history,chinese has so many words in the dictionary. I think,in chinese,但是、可是、不過...are almost the same meaning and all popular. The difference is how you pronounce them and your accent. I think japanese is the same way to express the meaning you want pp to know. Your pronunciation and accent will show up your emotions. So...In my opinion,these three words are almost the same. But if you know how to use them in a right way. Then when you're talking,even writing,you can be more "綺麗 desu"(←you know what I mean) It means your chinese is getting better as a native. Coz you know how to express one thing in different words. At the last. Though it maybe not the best answer but hope it's helpful for you. Best Rgds.