I was going to write as a comment, but it got too big and will as a response even though it is controversial that as far as I know
First it would be nice to understand the difference between Microdata
and Data Estruturada
This will help you understand the container and other information. Dynamic SEO works?
About the use of H3 it really has more semantic weight. Here is an article that will help you understand
"Heading Tags (H1, H2, H3, ...) are HTML programming features used to highlight titles and sub-titles of a page. H1 is the abbreviation of English for Header 1, or Header 1, thus the most important of the Headers."
Fonte:
https://www.seomarketing.com.br/heading-tags-h1-h2-h3-seo.php
OBS: It has been discussed a lot that each page should have only one H1 etc, but there is nothing concrete to confirm this "theory". Recommended reading https://webdesign.tutsplus.com/articles/the-truth-about-multiple-h1-tags-in-the-html5-era--webdesign-16824
Despite all of this, nothing indicates that Google uses them as a ranking factor, and they are more associated with semantics and accessibility than with SEO.
About the itemprop
This tag is quite new and it is part of the HTML Drafts, but is well documented by both W3C and Schema.org
See what Mozilla says about itemprop
"The global itemprop attribute is used to add properties to an item. Every HTML element can have an itemprop attribute specified ... and can be associated with a wide variety of elements including <audio>, <embed>, <iframe>, <img>, <link>, <object>, <source> , <track>
, and<video>
."
Source: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/itemprop
And as you can see in the Schema.org example itself they use the tag in a simple span
and again nthere is nothing indicating that they should be used in H1 or H2 or H3... https://schema.org/docs/gs.html#microdata_itemprop
<div itemscope itemtype ="http://schema.org/Movie">
<h1 itemprop="name">Avatar</h1>
<span>Director: <span itemprop="director">James Cameron</span> (born August 16, 1954)</span>
<span itemprop="genre">Science fiction</span>
<a href="../movies/avatar-theatrical-trailer.html" itemprop="trailer">Trailer</a>
</div>
Tip 1: Documentation of the tags used to build the Rich Snippet of Schema products https://schema.org/Product (no mention of headers)
Tip 2: Tool from Google to test your Structured Dates https://search.google.com/structured-data/testing-tool
Tip 3: Building Structured Date with the JSON-LD
, google article: https://developers.google.com/search/docs/guides/intro-structured-data?hl=pt-br (no mention of headers)
I’m not familiar with the attribute, but according to this documentation, it seems to me that the person who told you this is a little mistaken.
– Sam