We use the main section of any document for the main title and introductory paragraph of the document.

And Sub Sections

This is a sub section. Note that the header for the section is underlined with dashes. The dashes need to extend equal to or further than the text above it for it to be a heading.

And Bad Bad, Sub Sub Sections

We want to avoid three levels of sections like this.

This is a better Sub Sub Section

In other words, just use bold for a third level section.