Add numbering to the files that don't have it

Hello,
I'm trying to do the following:

1- Add numbering in the format 'xxx - ' (I know how to do this).
2- Change the existing numbering (I know how to do this).
3- But I want to make sure the numbering is only added if it doesn't already exist in the file names. I can't figure out how to do this part.

Here's an example of what I want to achieve:


001 - name01.mp3 >>> 002 - name01.mp3
002 - name02.mp3 >>> 003 - name02.mp3
ghjkname03.mp3 >>> 001 - ghjkname03.mp3

Thank you for your attention.

P.S.: I am using version: 3.77