While three days is more than enough time to see just the falls, there is more to Foz do Iguazu and the surrounding area that is worth exploring.
Arrive in Foz do Iguacu and get settled in, perhaps grabbing a bite to eat at one of the city's excellent selection of restaurants. If you have time, it takes about three hours to explore the Brazilian side of the falls properly; enjoy! Photographers should note that vistas over Iguacu are prettier in the morning.
Though the panoramic views over the falls from the Brazilian side are by all accounts best, Argentina offers the immersion experience, with several km of trails that thread the thunderous falls and crashing spray. Allow at least five hours to experience the Argentine side of the park.
Book a half-day tour to Itaipu Dam, considered one of humanity's most impressive engineering marvels. If you're bound for the beaches bound, consider grabbing a bus to Santa Cruz do Sul.