The certificate can be initially activated for the primary domain name only.
In order to add more domains, it is necessary to reissue the certificate in your SSLs.com account. You can change the primary domain to any other domain during reissue in Comodo (now Sectigo) certificates, starting from June 23d, 2016. Any other certificates cannot be upgraded to multi-domain ones as well.
It might happen that you have purchased the certificate with the default number of SANs and realized later that you need to secure more than 3 domain names.
In this situation you would need to purchase additional slots before certificate activation. The purchase is done from your SSL list > click on the certificate ID > click on “Add more” button in the “Secures 3 of 100 domains” line:
You will be redirected to the shopping cart where you will see the option to pay for additional domains. Once done, you are free to reissue the certificate with the extended amount of SANs.
Comodo (Sectigo) multi-domain certificates also allow to change the domain names in the slots already used during initial activation. You can simply type the new domain name in the available slot and proceed with reissue. The following guide will shows the steps on how to add domains during reissue.