I see that I can provide a variable username on the server node, but I'm running multiple scripts (3) and I need a different set of users for each script.
The easiest way i can see to handle such situation if you have only 3 different script is to duplicate your server.
Each server will have its own account variable. Then you can use the search and replace feature to replace at once the server of your virtual users.
You should check the NeoLoad documentation here:
I hope that will help you.