I had the same problem when posting a few days ago.
Though I think level 3 headings work for me if I use the markdown editor (e.g., a paragraph that only has “### How often have people been wrong about such things in the past?” will show up as a level 3 heading).
And when I just put a sentence fragment in a line by itself and in bold, it at least showed up in the sidebar as if it was a level three heading. (Well, one of them didn’t initially work, but then I fixed it somehow—I think the fix was simple, but can’t remember.)
I had the same problem when posting a few days ago.
Though I think level 3 headings work for me if I use the markdown editor (e.g., a paragraph that only has “### How often have people been wrong about such things in the past?” will show up as a level 3 heading).
And when I just put a sentence fragment in a line by itself and in bold, it at least showed up in the sidebar as if it was a level three heading. (Well, one of them didn’t initially work, but then I fixed it somehow—I think the fix was simple, but can’t remember.)