Batch to open a folder within a user's folder(c:\users\usernamehere\my documents)?

But I am looking for a code that will allow whatever user that uses my batch file to get to their my documents. The idea is that they would type 3, press enter, and then the program would open up their "My Documents" folder.

Unfortunately every user has a different name, so the following command won't work unless it has a specific name after it.

%SystemRoot%\explorer.exe c:\users\James\My Documents

Is there some sort of replacement for the "James"? A universal command?