3 hours is a very short period. I can't think of a single day in my life where I sat for less than 3 hours. I mean, I glanced at the paper and as far as I can tell that include eating breakfast, lunch and dinner (easily 1.5 hours right there). Even if I'm on my feet for the whole working day, I guarantee that I'll be taking a rest at night either playing a video game, reading a book or watching TV.
Saying that, I basically don't believe the study. The data was self reported and I literally don't believe that there is a statistically significant portion of the population who sit for less than 3 hours a day on average. Maybe I'm just misunderstanding the study...