We’ve done all three. Honestly I don’t think it would be a problem. There are a few places where the author says if you haven’t done Book 1, you can go to the course website and read terminology that is helpful. Once you buy the book, you have the code for the course website. We did do them in order, but you could definitely not. The order is based on Genesis, but each one is an independent book. Our favorite was Zoology 2, Swimming Creatures, but we loved them all.
Book 1 does spend some time on taxonomy, which she does not explain in as great a detail in the other 2. As someone else said, you could possibly read that first chapter if you have all of the books or perhaps you can just check the course website for helpful info.