I have found three custom filter sets that I like, but placing them all in inkscape's filter folder results in 50 scrambled filters showing up in the same dropdown menu under "Bundled".

The READ ME file in the filters folder states that "inkscape:menu" can be used to designate "the submenu under Effects > Filters".
I can open filter files in Notepad. However, I am not familiar enough with XML to know where exactly to "stick" this attribute.

I want the three custom filter sets to appear under their own sub-menus, so that I only have to scroll through a dozen filters at a time instead of all 50 of them.

 

It's just a couple of Inkscape attributes on each <filter> tag in the svg which contains your filters.

 id="defs1">
    <filter
       inkscape:label="MyButton"
       inkscape:menu="MyFilters"
       inkscape:menu-tooltip="Soft bevel, slightly depressed middle"
       style="color-interpolation-filters:sRGB;"
       id="filter4"
       x="-2.0275606"
       y="-2.1665932"
       width="5.0551212"
       height="5.3331863">

The first three attributes as shown above determine the filter name, submenu and tool tip for the filters menu (and the gallery).

If you don't already have a filters folder in your Inkscape user folder:

Go to Edit>Preferences>System.

Click open next to the user config folder. Make a folder called 'filters' and dump the svg containing the filters in that folder.

Don't put them in the main Inkscape program filters folder ( at ProgramFiles\Inscape\ etc on windows or /usr/share/inkscape on linux) 

That way you keep them portable and avoid any file permission errors later on.

Thanks!
I didn't think I could just "make" a new menu right there in the filter file.
I thought I'd have to somehow set up the new menu option in Inkscape itself as well.
The simplicity confused me XD