Learning to play a music instrument is not a race. Some people will conquer the skills to play very difficult pieces quickly while other people will need more time. At the end, it's not important how much time you need. What really matters is how well you learn.
The last few lessons of the Progressive method vol. 3 may be a bit difficult but you can always take more time to learn them. There is no rule that says that you need to do one lesson per week!
I've seen many people who had great intonation, sound and musicality but couldn't play very fast in the beginning doing great in auditions once they finally conquered those technical skills. They just needed a bit more time and their patience paid off!
So don't worry if some people ca play higher, faster and louder than you right now. In a few years, it won't matter at all. Just keep doing what you have to do and the results will come.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.