Laura, Generally it's recommended to enter FQDNs for 3 nodes installation. I kept it optional because this doesn't impact HE deployment. It's just a additional feature which will be executed after HE deployment success.
Laura, The hyperlink you provided in the additional information in step 4 of the document is not leading to the link and is throwing an error "404".
As the content of the documents looks good to me and the FQDN tab is present in gluster deployment (functionality is not working so another technical bug has been raised) so will move the bug to verified.