Dear Paul,
I suppose you are talking about Standard Accounts and what you purchased in our Marketplace are 3 Users? That is correct so far and with activating users you can log in with as many users as you subscribed to at the same time, provided you have installed the Standard Accounts client on several machines.
There are however 2 things you might have missed.
1. In order for you to be able to actually create 3 additional Users in Standard Accounts, also referred to as Persons, you need to subscribe to the Multipersons in app additionally (once for all users). You can do so in the Marketplace as well. This will enable you to create User cards for your 3 new users in System>>Registers>>Persons. Then you can create 3 new Persons with which you can then log in to the system
2. To be able to log in simultaneously from different computers you also need to be in the Cloud. Then your database is safely stored in the cloud and all your users can access it when logging in from their computers. To move your, as I assume, current local database to the cloud, please go to the Navigation Center panel (main window) and then click on the menu icon with the wheel. There you will find the option "Move to Cloud". After you've moved to the cloud, you can install Standard Accounts on 3 additional Computers and your new users can access it.
Best regards,
HansaWorld UK Support