I am in the process of migrating from MailChimp API 2.0 to 3.0
One of the features I used in 2.0 was to retrieve the HTML source from a template https://apidocs.mailchimp.com/api/2.0/templates/info.php so I could preview how a proposed email would look, without having to create a campaign first.
There doesn't appear to be a way to do this with API 3.0