Do a search for
(as cybmole showed)
<hr class="sigilChapterBreak" /><h3>Chapter \1</h3>
then after the replace, go back to top and remove the first
<hr class="sigilChapterBreak" />
if it's at the beginning of the whole text.
(i.e. there's no preface/prologue/contents etc.)
You can then use the Sigil feature 'Tools>Split on SGF chapter markers' (F6)
to split all those chapters into their own chapter files.