h1 for main title guy. And main title is decorated site name, or as you call it, logo. Here's the point -- in header of website, logo is not actually a logo, it's illustrated site's header. Designers just like to design it as logo.
What's wrong with LOGO ->
h1 is the most important section as it tells the visitor what's it all about, the visitor cannot understand the page, because
h1 is too specific. About who?
This page is about Google. Here's the section about us. See -- there's no question? All is clear.
Page structure. If you put site main heading into div or p, there's nothing to associate it with.
How can I associate "About U" with Google if
h1 comes after? Because all that comes AFTER
H1 is associated with it, not what becomes before.
"About Us" belongs to Google. No questions.
HTML is language to DESCRIBE information. Therefore you don't display information, you describe it. And each page is independent. Therefore you describe one page because there are no associations between pages. Just links that link individual pages.
THIS page is About us. Who/what it is? Unclear. It just describes something called "About us".
This site describes Google. And there is a section About us. Us = most probably Google, cause the structure describes it that way.
I hope I made my point :)
PS! You cannot use
h3 as it is illogical, therefore a big fail. Just because w3c haven't said it is not allowed, doesn't make it valid. It's illogical, think about it.