Useraccounts are not built to get chosen after the authentication has been done. It happens during login.
For your requirement, you can assign the userrole ‘operator’ to a useraccount and associate the useraccount to a location, usually during setting up her useraccount by some manager. When an operator logs in to the app, the operator can assign herself to one of the (three) available operator-positions at her assigned location. Which you can implement by associating useraccount to the location. Since your requirement is ‘select the operator name’, this requires an extra operator-entity.
You can set the number of teammembers, to limit the number to your required 3. And your other employees, the planners and admin, can get set and handled without interfering with the operator’s actions.