Pulling blog entries related to a structure entry field type
You can run a related to query, passing in all of the descendants of the 'parent' structure entry like so (assuming you are on the 'parent' entry page, else you'll need to query that entry first)

// Get the index pages children 
{% set descendants = entry.descendants.all() %}

{% set recentBlogPosts = craft.entries({
  section: blog,
  relatedTo: descendants,
  limit: 3,
  orderBy: 'postDate desc'
}).all() %}



Or, if you don't mind getting blog posts related to the parent page also, you can use the fact that in order to be related to a child page, a blog post must also be related to the parent:

{% set recentBlogPosts = craft.entries({
  section: blog,
  relatedTo: entry,
  limit: 3,
  orderBy: 'postDate desc'
}).all() %}