I want to change the title of my pages from H1 to H3, and if possible, make the title go away completely on the actual page.
I use the page titles for easy navigation for the user, so I can only do 2-3 words as the page title. For SEO reasons, I think it would be better to do a longer h1 title while still preserving the shorter page title in the navigation.
not doable with just CSS, it would require a child theme and rewriting the template files.
The Nav Label can be edited to be as short as you need it, without effecting the Post Title. I am not sure why you would want to replace a H1 with a H3, this in itself could be detrimental to your SEO.