但 is a simplified form of the 但是. -我想出去玩,但下雨了. be equal to 我想出去玩,但是下雨了. There is a little difference with 可是 and 不过 -他能胜任这份工作,可是他没有资格证书. behind 可是 often is negative result. -她肯定不同意,不过我可以尝试去说服她. behind 不过 often is positive result.

Brandon explains it well. http://studymorechinese.com/profiles/blogs/say-but-chinese-ke-shi-butt

i think 但 = 但是 = 可是 = 不過 我喜歡吃東西, (但) 我沒錢 我喜歡吃東西, (但是) 我沒錢 我喜歡吃東西. (可是)我沒錢 我喜歡吃東西,(不過)我沒錢 they can describe to "but" in English in the oral speak , they are less difference and the difference between them just leave them to the researchers because they cant tell very clearly even when normal people are talking 可是:it is softer way in talking, used in you want a thing very much 但是: it is the harder way in talking, used in what you want to strive for 但=但是 不過:i think it is similar like English of "however" basicly they are similar :D

just have some delicate diffarences, most time the some~

I think they haven't any obvious differences . 虽然你很好，但是我不爱你。 =虽然你很好，但我不爱你。 =虽然你很好，可是我不爱你。 =虽然你很好，不过我不爱你。