但是：[but;yet;however] 用在后半句,表示转折的语气 （如“虽然困难很多,但是他一点也不怕） 可是：相当于“但是” 不过：(1) [cannot be better] ——用在形容词性的词组或双音形容词后面,表示程度很高 （如：那就再好不过了） (2) [but;however;only] ——作连词,表示转折 （如：病人精神还不错,不过胃口还不好） (3) [only;just;merely;nothing but;no more than]∶副词,指明范围;只, （如：仅仅不过是个小孩子）

I think that 可是 is more colloquial whereas 但是 and especially 但 belongs to the written style and formal style. Or am I now far off?

但是 means that the sentence before it makes almost no difference! Though it maybe true. 可是 is not as strong as 但是, and we must say something useful in front of it. But, what follows it is of more importance! 不过 just equals however.

但是、可是的意思差不多，相当于BUT，不过：However...

Meaning almost the same, but 但是 contradistinction stronger than in case of 可是 ，and 可是 - stronger than 不过. Good luck.