Each App license can ONLY be activated for one ETS product.
You need an App for 1x ETS5 Professional and 2x ETS5 Supplementary. In this case you have to purchase 3 App licenses and activate each one separately to its related ETS5 product.
To activate your App follow these instructions: Licensing ETS5 & ETS5 Apps.
An App is always activated on an ETS5 product via its associated dongle. Once the App has been activated, you can NOT delete it or move it to another dongle.
Therefore, think carefully before choosing the dongle you need the App to be activated on.
After upgrading (e.g. from an ETS4 to an ETS5), all Apps assigned to the older version can be upgraded free of charge to the updated ETS5.
In this particular case please follow these instructions: App upgrade after an ETS upgrade