Include H3 headings in Table of Contents (On this page)

I want the Table of Contents (On this page) to include H3 headings. Currently, it only shows H1 and H2, which makes it difficult to maintain a deep document structure (e.g., 1.1, 1.2, 1.3) when the main points are set to H1. It is important because it helps users navigate complex articles more effectively.