Creative solution for Headings that sometimes have numbers and sometimes don't

Question

While setting headings for a book I ran into a typesetting issue where I would love to get some hints and different point of views from the creative folks out there.

I am setting a "book" that has the following TOC. Note that each major heading 1 has one heading 2 and each of the heading 2s have 2x heading 3.

1   Duis lobortis   
  1.1.  Quam non volutpat suscipit  
    Magna sem consequat 
    Ibero, ac hendrerit urna ante id mi.

2   Quisque commodo facilisis tellus    
  2.1.  Integer 
    2.1.1.  Sodales lorem sed nisl  
    2.1.2.  Morbi consectetuer  

3   viverra vita    
  3.1.  Boindi est ben  
    Ipsum dolor s   
    Dolor lorem

The special part is, that for clarity 2.1.1 and 2.1.2 are needed, where as all other heading 3 numberings can be omitted.

Note: In the real document there are more heading 2 entries like 1.2, 1.3, 2.2, etc. (they have been omitted for readability)

In the final book, the chapters are spread over many pages so that having heading 3 sometimes with numbers and sometimes not is actually not an issue.

What bothers me is, the TOC generated from these chapters, the TOC somehow looks very unbalanced, because 2.1.1 and 2.1.2 stand out:

Does somebody have a hint on what could be done here? Or even saying whether such scenario is more common that I thought? 😁

(I've tested either adding numbering to all heading 3 or removing the entire numbering on heading 3. Both options actually reduce the visual appeal in the chapters itself but make the TOC more consistent)

Answer 1

If possible, remove the 2.1.1/2.1.2 -- the other items aren't sub-numbered.

Or add... 1.1.1, 1.1.2 and 3.1.1, 3.1.2

With the missing leaders on 1.1 and 3.1 it looks like headings should be either sub-numbered (1.1.1/1.1.2) or numbered as primary items (1.2/1.3).

The numbering appears inconsistent.

I realize you've written you've tested altering numbering.. but in my view, consistency first, then balance is easier to find.