A compound SSL Certificate product available from some Registrars currently is the “Multi-Domain” Certificate, for up to 3 x domains by default, with more available from there.
To generate the required Key and CSR files to obtain those multi-domain Certificates, OpenSSL command lines like the following can be used:
openssl req -new -newkey rsa:2048 -sha256 -nodes -keyout FILENAME.key -out FILENAME.csr -subj '/C=XX/ST=STATE/L=LOCATION/O=ORGANISATION/OU=ORGUNIT/CN=FIRSTDOMAINNAME/emailAddress=EMAIL@ADDRESS/subjectAltName=DNS.1=SECONDDOMAIN,DNS.2=THIRDDOMAIN'
Additional domains can be added by appending more “,DNS.X=NEXTDOMAINNAME” (where X is from 3 onwards) in the subjectAltName parameter.
For more OpenSSL operations, see our older OpenSSL post.