Permission Inheritance

The root of all permissions in SharePoint is the Site collection. Permissions automatically flow downhill from the site collection (parent site).

Permissions should be inherited from the parent site whenever possible; do not break inheritance unless absolutely necessary as it creates administrative and security overhead/management problems and it also negatively impacts the performance of the site. It is also recommended to use Active Directory (AD) groups that will automatically update with the members that are added/removed to your organization/departments and assign the AD groups to the three different SharePoint groups accordingly.