There’s a subtle but important difference between a wildcard SSL and a multi-domain SSL. The former covers multiple subdomains of the same domain. Example:

blog.example.com
shop.example.com
intranet.example.com

The latter, multi-domain SSL, covers multiple, unrelated domains but is typically much more expensive. Example:

example.com
example.net
example.edu

But as @jarland said, letsencrypt should do what you want for free.

Cheers