inside div

Obviously there's no author mistake or anything:

<h3> tag can be in <div> without any problem. For some reason you just converted all the divs to <p>, and that's what caused it. Try to run this simple template:

~version: "2.1"

body: //article[has-class("type-post")]/div[has-class("share_box")]
<div>: //a[./img]
@remove: $body//iframe # Don't need iframes for the demo

@remove: $body//header
@remove: $body//section[@id="bottom-promo"]

Even with this code subheader works fine:

So it's clearly problem with your template.