In this time, Let’s Encrypt SSL certificates are issued only for an exact domain name. It means if you want to secure www.nuclear.hosting, nuclear.hosting and community.nuclear.hosting, you need to request the certificate for each of these domains and set up separately on webserver side. Of course, you may use SAN certificate (all domain names are included in one certificate). But, if you want to add another one, like my.nuclear.hosting, you have to request a new certificate (SAN or not).
Wildcard SSL certificates allow you to secure all subdomains. Wildcard certificates are issued for *.domain.name (ex. *.nuclear.hosting), so you can secure all subdomains without requesting new cert every time when you add a new subdomain.
Free trusted certification authority Let’s Encrypt annouced future support of wildcard SSL certificates, planned on January 2018. Full annoucement you can find here.
Wildcard certificates are one of the most demanding features and will be available from January 2018 along with the new ACME v2 API endpoint. Let’s Encrypt hopes to make it even more attractive HTTPS. Wildcard certificates will be issued free of charge.