@Erymnd IIQ sends reminder based on certification. In your case, your must have one Certification assigned to the manager and that certification has 3 different certification items for each of three users.
You need to modify your template and call a rule to add reportees names and access details. Please refer to this post for reference: How can we pull the Display Value of an entitlement into a custom Remediation Notification email template in certifications? - #10 by neel193