Is this documented somewhere? Are you sure? How can you tell that you actually have 3 dynos running when you're paying for 3?
I suspect that if you pay for at least 2 dynos what happens is that they keep at least one dyno running and scale up to 3 as needed because as a customer I can't really tell the difference between 1 and 3 dynos unless my traffic requires all 3 at which point the service would have already scaled up to three. We already know from the free tier that they can spin up an instance in a couple of seconds.
I would bet this is Heroku's little secret about making money. If you look at their cost per month you find that they aren't really making much per instance on AWS servers although, they are perfectly capable of detecting when they should auto scale up. Amazon even has a built in service to do that, but Heroku doesn't expose it because that's where they make their profit margin.
I can say from experience they definitely do not scale down the dynos you are paying for - you still see log entries from them and they are immediately available to requests.
I suspect that if you pay for at least 2 dynos what happens is that they keep at least one dyno running and scale up to 3 as needed because as a customer I can't really tell the difference between 1 and 3 dynos unless my traffic requires all 3 at which point the service would have already scaled up to three. We already know from the free tier that they can spin up an instance in a couple of seconds.
I would bet this is Heroku's little secret about making money. If you look at their cost per month you find that they aren't really making much per instance on AWS servers although, they are perfectly capable of detecting when they should auto scale up. Amazon even has a built in service to do that, but Heroku doesn't expose it because that's where they make their profit margin.