How to Replace Last three slash from the url with comma(,)

Question

I have a url

 

string url="http://www.xyz.com/forums/AddPost/NewPost/Hello";

 

I want to replace last three slash(/) from the url with comma(,).

 

I want url like this 

 

http://www.xyz.com/forums,AddPost,NewPost,Hello 

 

Please Solve my problam.