Tag from filename - using 3rd Directory

I'm having trouble writing a filter script that will help me pull the artist from my directories.
I've got "D1/D2/Artist/....filename.mp3". There are various numbers of directories between the artist and filename.

Is there anyway to identify the 3rd directory in filter script /reg exp?

I think I do not understand the problem.
You can filter for data that is already there and can be named or for the total absence of a particular piece of data.
But somehow I understood that you have the artist's name burried somewhere in the path and you want to "pull" it somewhere.
So yould you be as exact as possible about the path structure as it is almost impossible to create an exact solution on the basis of vague descriptions.
So, could you try it again in other words some more details?

Maybe you can adapt something from there ...

You can simply solve your problem using this filter expression ...
"%_folderpath%" HAS "\Bandhouse"

... or use a regular expression to split the folderpath into folder components ...

"$regexp(%_folderpath%,'^.+?\\\\(.+?)\\\\(.+?)\\\\(.+?)\\\\.+$','$3')" IS "Bandhouse".

Here the components are ^Drive\Folder1\Folder2\Folder3\Remainder$

Note: The filter does not 'pull' something from anything, but restricts the list view for the currently loaded files, fitting to the filter expression.

If you want to 'pull' a partly value from the given file's folderpath, then you have to apply a scripting expression within a converter or action dialog.