Just specify the same S3 account for all servers and you're good to go. Backups are organized in the bucket based on the computer prefix, which you can manually override in the S3 properties in CloudBerry Backup in rare cases where they are not unique.

thank you! Do I install the same CloudBerry executable on all servers, then?

If you're using our stand-alone product then you'd need three licenses, each installed on a server. You can export plans and setting to make it easier to copy those to a new server. If you'd prefer to use our Managed Backup Service, you can purchase monthly or annual subscriptions and manage them all from a single thin client interface.