After third post, on blog page template

Have you already checked available positions for automatic insertion using debugging functions?
https://adinserter.pro/documentation#debugging

I have and it does not show any available positions for automatic insertion. Is that typical with a Genesis blog page template?

Thank you!

Hard to say if it is typical, looks like some custom template is used.

If your theme uses Genesis Theme Hooks
https://genesistutorials.com/visual-hook-guide/

then you try with custom hooks
https://adinserter.pro/documentation#custom-hooks

And as a last option you can try to use insertion before/after HTML element:
https://adinserter.pro/documentation#insertion-before-after-html-element

Try to insert after element article.post
White-list also url /blog/ so this will apply only to the blog page.

The easiest way to limit insertion only after 3rd post is to use Server-side insertion (you need to enable Output buffering for this) and then set Filter to 3:
https://adinserter.pro/documentation#filter