DATE regular expression apply

The DATE field is usually made up of 4 digits
mmgg 0405 (04/05/2021)
I have files that mistakenly show me 3 numbers (405)

how can I set a regular expression that would detect if the DATE field has 3 numbers, add the 0 zero in front.

normally in notepad I would do this:
search: (^ \ d {3} $)
Replace: 0 $ 1

here maybe I should do:
(^ \ d {3} $)% date%

but clearly it doesn't work because I don't know how to regex the tag fields

It looks to me like you are already accustomed to regular expressions.
So a look at the help for an action of the type "Replace with regular expression" may be enough assistance:
https://help.mp3tag.de/options_format.html#regexp

yes Ok I got there too, the problem that I can not understand how to apply it in reality.
Can I link some examples on the forum?
I already use the regexp in general but here I can't understand how to put them into practice since in the common use that I could do with NOTEPAD ++ my regexp works, but applied in Mp3Tag the same does not work, so there must be some application difference that in that guide it is not written or it is not clear to me how to do it.

I tried to search the forum but nothing.

could be: ^(\d\d\d)$

should be: 0$1

Test your expressions in Converter>Tag-Tag, not in Notepad