That would be 9 hours per day, every single day of those three years. How many people have even had just one day where they spent nine hours on the same thing? That's not including breaks, and periods where you mind wanders or you're not focused.
And it is supposed to be 10,000 hours of "deliberate practice", if I remember correctly, i.e. exercises and trials focusing on harder point, with an immediate feedback. I don't think it is possible to keep that kind of focus 9 hours a day for 3 years in a row.
