What I really want to do is find a way to make Word display in the header
whatever words I have marked as "Heading 3" on that page. So, let's say I
have the following:

Line 24: "I love you long time." (marked as "Heading 3")

I want the page that line 24 is on to display "I love you long time." in the
header. I want this to self-update, so that whatever is marked as "Heading
3" on a given page will display that same thing on that page's header. Think
of it like a dictionary page: if this was a dictionary I was making, I'd mark
the first and last words on the page as "Heading 3" and want the Header to
read "First word - Last word" and to change for each page.

How can I do this?