Hello.
I have recently purchased Avast Internet Security for 1 computer but have recently purchased a computer more. Is it possible to upgrade from Avast Internet Security from 1 to 3 computers or do I pay for a new license?
Sincerely Britt
Welcome to the forum. 
May I suggest that you address this question to “sales @ avast dot com”.
There isn’t a way to do it from the standard license purchase as far as I’m aware. You could contact sales (at) avast (dot) com and outline your wishes, with information on your existing license and its expiry, etc. hopefully they can organise a custom arrangement.