Hi Here is your Solution,
string str = "1234567";
Regex rex = new Regex("^[0-9]{3}");
Match m = rex.Match(str);
string FinalString = m.Value.ToString() + "/" + str;
MessageBox.Show(FinalString);
Even you can do this without Regular Expressions too m using simple codes ..like substr() function
if any problem arise let me know :)