I love having the option of an H3! I think the styling of the header sizes is good. It’s distinctive enough to see immidiately where in the hierarchy it is.
One thing that could make it more flexible would be to allow it headers to be bolded. I realize they are already bold, but marking a header as bold now doesn’t make anything happen (unlike underlining or italicizing). Maybe bolding a header could make the font weight even heavier, equivalent to black.
Also, it’d be great if the headers weren’t hidden on larger viewports. See the screenshot from how it looks on the screen I’m working on… A lot of extra space to the right of the toolbar yet the header and justification buttons are hidden in dropdowns.
It’s not at pretty to have these visible I suppose, but for poor souls like me who are forced to swich between Windows and macOS everyday it’s easier to just click than to hurt my brain by trying to learn the keyboard shortcuts for these commands on two platforms…