WordPress content on non-WP site residing on a remote domain — how to display posts?

You can grab an RSS feed by going to /feed/, adding /feed/ to the end of any WordPress archive or page will give you an RSS feed.

You can then grab this remotely and display the first 3 items in your sidebar.

However, your sidebar is inside a non-WordPress site. So the styling, html, how to grab an RSS feed, etc, are all off topic here, as they’re non-WordPress questions