I was just wondering, if I were to pay for one of the Avast 3 year subscriptions does it only go for one computer or can i use it on multiple computers?
I believe it’s 1PC for 3years. 
The usual one for the Avast Internet Security suite is 3 systems one year.
You can however, change the license for more years 2 or 3 for 3 systems. It is relatively flexible but you would have to evaluate which gives you the best value. There have been times when the three year deal wasn’t as good as the 2 year deal on a cost per system basis.