Usually your post title is the h1 and the next headline inside the article should be h2. There can be a few h3 under a h2. But these should be in order. Like you should not start an article title with h3 and put h2 inside the content.

H1 article headline
- H2 subheading
- H3 minor heading
- H3 another minor heading
- H3 third minor heading
- H2 a new sub heading
- H2 more subheading