What's the difference between 对不起, 对不住 and 不好意思
对不起(*): Sorry/Excuse me

对不住: Ditto (colloquial). I'd assume it is mainly used by northern Chinese

不好意思(*): Excuse me (colloquial)

* Other meanings exist in different contexts, e.g. 对不起=let somebody down, 不好意思=be embarrassed, etc.