Hi Lillian,
There is a report under Notification Server Management\Agent called "Agents Installed in the last N Days". You could set the N value to 3, or clone it and then set the value to 3. The report can then be used in an Automation Policy- you could create a new one, specify report as the data source & under actions select send email task- Jobs & Tasks\Samples\Notification Server. You'd also need to configure email settings for NS under Settings\Notification Server Settings\E-mail. Main thing is to have an SMTP server to specify, surely an organization has serveral at its disposal .)
I hope this helps, let me know in case of any questions.