It only takes one shot at the right time to protect a puppy from parvo. So why 3-4? It is easier to give a shot that tell if the time is right. A shot won't take until immunity from the mother wears off. What the puppy needs is one shot between when the immunity wears off and 2 weeks before it is exposed to parvo. Best thing is to keep it away from anyplace frequented by unknown dogs.
For a more complete and authoritative explanation see
Canine Parvovirus, What you should know about I trust the AVMA's site more than one somebody Googles up. Tons of sites out there with advice ranging from useless to dangerous.