civiCRM Cron only run one mail_report scheduled jobs?
You have to create 3 instances of the mail_report scheduled job.

To do that, use Add New Scheduled Job button on the Settings - Scheduled Jobs page (civicrm/admin/job?reset=1)

On the creation page, set API call to Job / mail_report (api.job.mail_report) and define in Command parameters the appropriate instanceId and format options.