Since Server Name Indication technology is supported by Plesk, each site may have their own SSL certificate even if they reside on the same IP. If no certificate explicitly selected for a domain (Websites & Domains > click on a domain name), then default certificate of IP (Tools & Settings > IP addresses > click an IP address) is used. This will be "default certificate" if nothing else is provided.
My guess is that there must be an IP address with "default certificate" as default and there must be 3 domains on that address which don't have any other certificate selected.
If it won't help you probably need to contact support service for database scan and discovery which domains may be using this certificate.