by accident,by chance &happened都有啥用法和区别啊?讲讲吧

by accident 和by chance没有区别.
happened to do碰巧做某事,这是个动词句型,而上面的两个是介词短语.
请看：I met a friend of mine in the street by chance/by accident yesterday.=I happened to meet a friend of mine in the street yesterday.