That's such a contrived example why would we even bother with it?

Typically when people purchase or obtain items that they need to transport home or to someplace, they do some sort of planning ahead. Also, when I was a kid and needed transportation I typically could just ask an adult to help out with this. Or, another option would be to ask the school to hold the object until you could arrange transportation.

In any case, three miles is not a particularly long distance to traverse, and as you pointed out above you were actually able to walk that distance while carrying it. Well laser printers are somewhat heavy, I'm sure it was a little annoying to carry and you probably had to stop several times and rest. By myself had to carry 30 lb of lumber about 3 mi home from the lumber store years ago when I didn't have a car. And yes I was able to manage.

However, cargo bicycles exist and are quite popular in many parts of the world including the United States and can carry up to 400 lbs, so it is not true that the only means of transporting moderately weighty items is by automobile.

In fact, I would be willing to bet that you would not even need a cargo bicycle but a normal bicycle with a rear rack and a couple of bungee cords would probably suffice. I know this because I got away without having a bicycle for 6 years as an adult American in a city and even went bicycle camping a few times with my friends.