So, I want to read file from stdin, delete all '/' in line that contain exactly 3 '/', and write the output to stdout. So a file contain:

```
/a1/b/c
/a/b2
///
/a

```

will have output:

```
a1bc
/a/b2

/a

```

I am thinking something like this:

```
sed -r 's/\/[^\/]*\/[^\/]*\/.*/"I not sure what do I need to put in here"/g'

```

however, I am not really sure what do I need to put in the replace session.