How do I set the default portal platform to Version 3 for certain security roles?

While most users use the Portals 3 platform, and it is highly recommended that you do, some may opt to use the Portals 2 platform.

The Default Portal Platform field on the Security Role detail screens sets the default portal version for everyone with this security role.

The default portal platform is set per security role on the Security Role record, which is accessible through the Security Roles query on the System homepage. The Default Portal Platform field allows you to choose between versions 2 and 3 of portals.

You can find additional information about Portals in Learn Veracross.