I've read the FAQ but am not sure how to do this. I want to insert a space after the third character of every filename.
It may be easier to recreate the filename from the tags with the function Convert>Tag-Filename.
For the initial problem:
Create an action of the type "Replace with regular expression" for FILENAME
Search string: ^(...)
Replace string: $1
(as the blank behind the $1 would not show, I used _ - please use the space character instead).