I do professional development on company time. Our company provides $500 to spend on books, classes, conferences, etc, with approval from your supervisor. Anything I buy from that I assume it's fine to do on company time. In generally, I spend a half an hour a week on PD/networking outreach per week. I have it blocked on my calendar. So usually, I'd just read the book, and it would take me six weeks. If I needed to read it more urgently, I'd condense into a week, and block out like an hour 3x to read.
I wouldn't do three hours straight of something like that on company time. But I FOR SURE wouldn't read for three hours on a weekend when the request came from by boss!