If one pc runs 2 virtual machines with their guest OSes, does it count as 3 pcs or 1 pc in terms of the license?
IOW, can I use the same license there on both vmboxes or do I need to get 2 new trial-licenses for this test?
You used a link to a question about the FREE version of avast.
Paid versions have different rules. They also have licenses that cover multiple computers and multiple years.
A single license for one year covers exactly that.