The question of how long immunity to canine distemper, parvo, and rabies lasts after vaccination is a good one.
At my first job as a veterinarian in upstate NY, over 40 years ago,distemper was still occasionally seen in owned pets. My boss told me 'you see distemper in two kinds of dogs - puppies, and old dogs who were vaccinated as puppies and never got any boosters.' (These were different times- that was a large population.) And I respect the experience of a long time practitioner over untested theory.
Clearly the protection lasts well over three years. But apparently it does run out eventually. (Compare to tetanus in humans.)
So why don't they just find out? Sure, all you have to do si keep a colony of a couple hundred beagles at a university for 10 years for the sole purpose of drawing blood from them once a year. Of course, there are other experimental models, such as titering all dogs that come into a large group of hospitals and comparing titers to time since last vaccination, but these are similarly impractical. Not very many dogs come in from responsible owners who haven't had a vaccination in 6 years.
Add to this the fact that, in practical terms, if you set 'reminders' at much more than three years you may as well not have any boostering plan at all. People move, they change veterinarians, heck in 7 years most veterinarians have moved on for one reason or another. If you say 'We will booster this dogs vaccines every 7 years', you are saying 'We will not be boostering this dog's vaccines' for the majority of clients.
Putting that all together, it would appear that a three year schedule is reasonable, not onerous, not dangerous to the patient or overly costly, and a fair way to keep these horrendous diseases right where we want them... gone.